Let's just put it this way: he takes only a couple of hits to fall and becomes a regular type of enemy later in the game. This dinosaur's not even worth being a boss.
The Wyrm has a Slurp skill that drains fuel and is strong against regular attacks, but with a couple of Guided Shots he's dead. You can also use combos if you feel like it.
Bart's gear has that annoying Wild Smile skill that damages your camera (reducing your hit rate), but it's temporary. Use booster if you want, then Triangle-Square attacks and Guided Shot to finish him easily.
Before anything, make sure Weltall and Brigandier are tuned up with their best parts. Have Bart use Wild Smile to reduce Calamity's accuracy (He's already a tad slow), then use booster and pound him with Cross, then Triangle-Square (or Triangle-Triangle) attacks. Beware of his Rockets, they do lots of damage, especially to Brigandier.
Gebler Units (Swordknight, Aegisknight, Wandknight (2), Clawknight, Schpariel)
You don't have much of an option against the knights, but they're fairly simple: just keep attacking them. The Schpariel has a stronger attack called "Ground Breaker", but it's still no big deal.
Gonzalez, Big Joe, Scud, Dan, and Wiseman
Those are the tournament enemies. Gonzalez is no big deal, except he will counter-attack if you use a Deathblow. Just build up your AP bar with weak attacks and finish him. Big Joe is constantly supported by his fans (literally :P), but filling your AP bar works perfectly too. Scud can be defeated either with the AP bar combo, or with a couple of Guided Shots. Lastly, defeat Dan using any Deathblow and heal if your HP gets too low (his best attack takes away around 80 HP). And no, you can't beat Wiseman, he'll just evade your attacks (except for Guided Shots).
Use any Deathblow or build up your AP bar to attack him. Ramsus will be healed often by Miang (and so will you, by Margie), but don't worry too much about it. The only thing you must NOT do is attack Ramsus when he uses his "Mirror Stance". Although he has more HP in the second battle and Miang is more efficient, Fei joins the crew, so it should be equally easy.
Gebler Units Again (Wandknight (2), Aegisknight, Swordknight, Clawknight)
They have the same status from the last battle. The Wandknights' "Power Beam" attack is easily reduced if you equip a Beam Coat. The second battle can be a bit annoying, but if you defeat Swordknight early, you'll avoid a big loss of HP (and you should, because of the next battle). Finish Clawknight, then Aegisknight with the usual Cross, then Level 1 or 2 combos.
I hope you survived the last two battles well, because this one is the toughest. Elly has a decent evade rate, avoiding many Cross attacks, and often uses her Aerods attack for high ether damage. Have Fei (not the gear) equip an Ether Veiler to raise his elemental defense, activate booster, and use your best combos on Vierge.
Main Gun (Keifenzel), 2 Small Guns
Ignore the small guns and try to destroy the main gun before it hits you (the small guns will be automatically destroyed when you destroy the main gun). This an easy battle, but you shouldn't waste too much HP or Fuel because of the next one...
At the start of the battle, you won't be able to damage Dora; keep attacking it with Weltall until the Head Cover is blown, then you can use the Pirate Gears to help. Dora's cannon does some decent damage and he'll pick up your ally gears and use them to counter-attack you for more damage. Use booster and remember: you can win this one even if Fei is down! (Go Falkon, Maitreya!)
Just a reminder about this boss: there's no way you're gonna defeat it :P.
Leonardo, Heinrich, Vargas, Suzarn, and Rico
Just don't forget to save between each battle. The first 3 guys are easy (just use Iron Valor and Inner Healing when necessary) and Suzarn has some scary special attacks (like taking half of your HP or taking all your EP), but her regular attacks are weak, so use Aquasols if you need any and attack her. Rico is unbeatable; just wait until he knocks you down.
This boss can be really nasty if he insists on using his "Murder" skill; you must have lots of Zetasols in case he uses it. Cast Iron Valor and Steel Fist and build up combos to finish him quickly. The Bloody Rain can be avoided if you have Citan cast Renki, then Suiseii; the other status-altering attacks he uses can be blocked with the items you got in the sewers (Poison Guard and Cool Shades). Also, having Citan casting Renki and Sazanami will heal all characters (you might have to do this if he kills Rico or Fei).
You can beat him this time, but he only gives 1 exp (d'oh...) and it doesn't matter if you win or lose. He'll end the battle using a technique similar to yours.
Looks identical to the first boss and is probably even easier. Seriously.
Gebler Units with an extra "R" (Aegisknight R, Swordknight R, Clawknight, Wandknight R (2))
This time they're a tad stronger, but not enough. The Swordknight R might do some serious damage and so do the Wandknight Rs with their beam attacks (equip a Beam Coat if necessary), but it's still no big deal.
Hetch and Super Aerod
Dominia can be damaging to Weltall, but Stier is quite strong against her regular attacks. For some weird reason, Beam Coats works well against her Ether attacks. Turn on booster and ignore the Super Aerod since she'll summon another if you destroy it and Hetch won't counter-attack (or attack) while the Super Aerod is active. The damage dealt by the Super Aerod isn't that much for someone who just recovered all your HP, but in any case, use Frame HP when necessary and keep pounding Hetch with Cross, then Triangle-Square combos.
Fis-6 and Fis-6 Mechanics
Activate booster mode and start pounding him. The more you attack him, the stronger Fis-6 gets. After a while he overheats, lowering his stats, and some mechanics come to heal him. Kill the mechanics and repeat the process.
So you think it's an easy battle because you're riding a Gear and he isn't? Not that much. His Super Guided Shot halves any Gear's HP, and your only advantage is that he refuses to attack Vierge for some reason. It's not worth defeating him, though, as you only get 1 exp.
If you upgraded Brigandier and Vierge, this battle is easy. If you're too slow, equip an O2 Cylinder or a D Circuit and equip a Ground or a Beam Jammer to reduce Electric Damage.
Yes, it's Dominia again...she keeps changing her elemental attack and moves quickly, but don't worry, just use your best attacks because you're warming up for the next battle.
Haishao is back but Ramsus is piloting it and Miang is backing him up. Cast Ethers like Anemo Burn or Thermo Dragon with Elly (works wonders with an Ether Doubler) and Wild Smile with Bart and keep bashing him. Like every battle with Ramsus, you must beat him fast or Miang will heal him, and NOT attack him when he uses the "Mirror/Predict and Counter" pose.
A weaker version of Redrum; He starts quite often by using his 'Murder' skill, but after that he stops using it for a while (and he's also much slower compared to Redrum). Use Zetasols or Billy's healing skills and take him down with your Deathblows (I didn't even build the AP bar for this one).
Initially, you can only hit him with Billy's Square attacks or by casting Ether (any Ether that you cast on him, not being necessary to bring Elly). The problem is that every time you hit him with an Ether Attack, he raises his attack but lowers his defense; I'd recommend having Booster on and have someone cast an Ether every turn on him until he's taking lots of damage; Have the other two use Level 1 or 2 Deathblows and he'll be dead before he can do any damage to you.
Tolone & Seraphita
If you have Elly on your party, have her attack with the Lunar Rod every turn on Seraphita since she can deal more damage to the party and steal EP (the Lunar Rod has a chance of putting enemies to sleep). Have Citan cast Renki, then cast either Suiseii (against Fire, if you prefer to put Tolone to sleep) or Chiseii (against Wind). Unleash 28 AP combos until they're defeated (watch out if you leave Tolone for last because she has an attack that does lots of damage).
Having Billy in this battle helps a lot. Id can do lots of damage each turn (sometimes he attacks twice a turn), and sometimes he concentrates all attacks on one character, which will most likely kill characters like Elly or Billy. He can even evade Deathblows. Luckily, he doesn't have much HP, so have Billy heal every turn and everybody else attack him with your best Deathblows/combos.
This a long but not so difficult battle. When the battle starts, you cannot hit him, so keep charging. After his barrier is destroyed, do the usual Cross and Level 1 or 2 Deathblows sequences. His attacks aren't too dangerous, except for Mass Limit (halves your HP) and Seal (if you don't have Seal Ar). Use Frame HPs when necessary, keep an eye on your fuel (you will waste most of it) and watch out because he can take a lot of damage before he dies.
Use the same strategy from the previous battles against him. Just don't attack him while he's using the Mirror Stance and use your best attacks to prevent Miang from healing him.
Use Ether Attacks and your best Deathblows against it. Seibzehn has high defense, but it's rather weak when compared to all your gears together.
6 Solaris Guards and 2 LittleFoot Gears
This is an easy battle if you have Vierge (if not, you'll take lots of damage from the Guards). Use Aerods or any Ether against all enemies (helps having an Ether Doubler equipped) and the Guards will be down in one turn. If the gears survive, destroy them with regular attacks.
3 White Knights
Depending on how high your defense is, these Knights may not even hurt you. They're rather quick and evade many attacks, though. Turn on booster to keep up with them and use regular attacks and Deathblows to finish them.
2 White Knights and Citadel
Have your gear equipped with the Silver Ar (or Silver Ar +1) that you got after the first generator battle to reduce the damage from Gel Breath. Turn on booster, destroy the White Knights (you can optionally leave them for later if they can't damage you), then concentrate on Citadel. Watch out for the Sonic Boom Attack, since it does lots of damage.
2 White Knights and Avalanche
Equip your gear with an Ar Repairer (I hope you bought one at Shevat) and the Gold Plate you just got from the third generator battle. Turn on booster, kill the White Knights, then attack Avalanche. If you don't have an Ar Repairer, you could receive over 5000 damage from one hit! The Gold Plate will prevent any Gel Attacks, but you can't escape the Sonic Boom Attack. Just use a Frame HP if things get too rough.
With Chu-Chu, just use any regular attack until the battle is over. With Seibzehn, use your best attacks and Deathblows (you can use Level 3 Deathblows if you're curious) until Achtzehn is down. Achtzehn has only one decent attack, but I doubt you'll need to heal or turn on booster to defeat him.
2 Neo Etones
This is just a battle for you to test your first Omnigear, Andvari (these gears become regular enemies later on). Even the Neo Etone's Seal is weak compared to your gears' HP. You'll be done in a couple of turns with regular attacks.
Have Seibzehn and Andvari for this battle (these two gears have the best equipment at this point) equipped with Seal Ars (or Seal Barrier) and WaterVeil Ars, then use booster. When Shakhan connects to the Gate, he heals himself every time you attack him. When he disconnects, you'll be counter-attacked each time you hit him. At the start of the battle, attack him with regular Triangle attacks (it doesn't matter if he heals, just use it to raise your Deathblow level to 3). After Shakhan disconnects, unleash your Deathblows and use the Cross, Level 1 Deathblow sequence afterwards. Stop attacking him when he connects again (I'd suggest you turn off booster for now) and wait patiently (you don't want him to heal all the damage you did, right?). Use your best attacks when he disconnects and repeat the process. This a tough battle, but you'll eventually win.
Bladegash and Marinebasher (Twice)
This battle should be easy, except for the fact you'll have to beat them two times with no chance of recovering HP or fuel, so be aware of this when using booster. Any equipment to reduce Ether Damage helps (Ethers Ars, Ether Veiler against Bladegash or FireVeil Ar against Marinebasher). Equip Elly with a Flare RodG and attack Marinebasher (you could also cast Thermo Largo if you equipped an Ether Doubler) and have Fei help her. Against Bladegash, have Elly cast the opposite Ether to her Elemental Sword (i.e., if she uses Water Sword, cast a Fire Ether) and Fei use his best Deathblows. Remember, the secret to winning is finish the first round with enough fuel and HP left for the next battle.
Grandgrowl and Skyghene (Twice)
Basically the same problem as with the previous bosses, except this time it's easier (you can even turn booster on both battles this time). Have Andvari and Seibzehn (You didn't choose Chu-Chu, right? :P) pound on Skyghene, then on Grandgrowl. Repeat the process on the second round to finish them.
Pretty much like the Seibzehn battle, just pound her with your best attacks until Crescens is down. She has an annoyingly high evade rate and some decent Ethers, but that's all.
Grahf and the Executioner
The Executioner has lots of powerful Ethers that hit everyone and an instant-kill attack, so concentrate on her first. Have Billy heal every turn and Citan use his best Deathblows. Equip someone with a Power Crisis if you want to do more damage, but it's risky.
You already start with System Id on, so attack him with Goten X (Square button) three times and he should be defeated. Vendetta can do quite some damage, but unless you're really unlucky, this is another easy boss battle.
You can't lose this battle (unless you really want to wait and see what happens if you keep defending the whole battle). Either kill it with regular attacks or use Yggdrasil's cannon.
Sufal Mass and 4 Sufals
You cannot hurt Sufal Mass with elemental Deathblows or Ethers, so bring characters that use non-elemental skills (like Rico, Emeralda, or Billy). Kill ONLY three Sufals and ignore the fourth one. Attack Sufal Mass with your best non-elemental Deathblows and heal when necessary, then kill the last Sufal after Sufal Mass is defeated.
Leave Dominia for last; defeat Kelvena with fire Deathblows since she's the party's healer, then take out Seraphita with water Deathblows before she steals too much EP; beat Tolone with earth Deathblows and Dominia (you can defeat her before the other girls, but she'll heal herself for 5000 HP if you get near it) with wind Deathblows. Alternatively, use Emeralda/Elly's Ethers to put those girls down.
Have booster on and use your best attacks (System Id, Infinity Deathblows, or Ether according to each girl; you can check which girl is on the lead when any of them says something and powers up), and note this battle may take most of your fuel, but don't let your HP get too low! There's a good chance G-Elements will blow up and damage your party after being defeated.
Hammer grows stronger after each transformation, but it's nothing too dangerous. The problem is that when he turns red, he'll self-destruct a couple of turns after, damaging the whole party. You can escape if you want, but if you manage to destroy him before he explodes, you get his experience and the awesome Trader Card. I recommend having Fei using System Id and the two others characters (pick fast characters like Citan or Emeralda and give them Holy Pendants; cast Yamiga on everyone to raise your attack power) keep attacking, building up their Attack Level to 3 or Infinity. When Hammer turns red, use the Infinity Deathblows you've been saving and Fei's Kishin (you have to be at level 70 to use it). If you're lucky, he'll be dead in two turns and you'll get the Trader Card.
Deus is quite easy if you know what to do. Attack him once when the battle starts (he'll heal himself, but he hasn't damaged himself yet, so it doesn't matter), then let him half everyone's HP and wait until he drops his own HP to 5000 (Use Frame HPs while waiting if you want). Cast Yamiga or Yin Power to raise Weltall-2's attack power and have him use Kishin and the others use their best attacks.
This is an almost impossible battle depending on your luck, unless you're using GameShark codes. The funny thing is that he drops the Slayer Robe (rarely) if defeated, which is one of the most powerful defensive equipments in the game. Anyway, you don't have to defeat him (and it doesn't change the scenes afterwards), but if you want to, here's my strategy: first of all, you must not have Citan in your party when you fight Deus, or else only two gears will fight him; second, equip anything that raises your response, like Response Circuits or Magnetic Coats; third, have Bart cast Wild Smile six (or more) times. If you survive the first few turns, there's a good chance Alpha Weltall will miss almost every attack and slow down a lot. Turn on booster and start attacking him with your best attacks (Kishin, Infinity Attacks, Ethers, etc). Note that if he kills Bart right away, you should just reset and try again.
This is a rather tough battle, mainly because you can't save before the next boss battle and Amphysvena starts the battle using Wave #3 Demiser, which reduces everyone's HP to 1. If you restore everyone's HP, he'll use Wave #3 Demiser again after a couple of turns, so you should just destroy him as quickly as you can (Booster, System Id, and constant Deathblows help). If you don't restore one of your gears' HP, Amphysvena won't use Wave #3 Demiser again, but you should still watch out (don't worry if he kills the gear with 1 HP; the same gear will come back for the next battle with a bit more HP). Also, it is recommended that you finish this battle with as much fuel (and HP) as possible, since the next boss is a killer.
Have everyone equipped with Ether Ars +3 and refuel a little. Opiomorph gets stronger with each hit you do, and as a consequence if you do lots of damage, you'll receive even more damage when she uses Wave #4 Devastator (after she uses it, her status will go down, though). Depending on your Ether Defense with Ether Ars +3, Opiomorph might not even damage you if you keep refueling. Take this chance to restore your HP and fuel, and start attacking with weak triangle attacks to slowly receive damage. Note that when Opiomorph's HP gets too low, she'll start raising her Ether power no matter what happens, so turn on booster and use System Id in this case. I suggest holding back until you have two gears with Level 3/Infinity Deathblows and System Id ready to attack, then try your luck. This battle can be quite difficult, but you'll eventually win.
Id (True Form)
Easy, even without your Omnigears. Just use regular attacks and Deathblows until Id is down. I didn't even use Frame HPs once during this battle.
Even easier. You lose System Id, but you have 30000 HP and a really good chance to get Level Infinity attacks. You may have to use Frame HPs once, but it never happened to me. Just get rid of Grahf for good.
Harlute can freeze two of your characters and causes lots of status reductions, like Def Down, Atk Down, etc. Except for his Heavenly Annointment, his attacks aren't that strong, it's more of a question of patience. Use regular X, Level 1 Deathblows (or raise to Infinity if you have Holy Pendants) and take him down. Booster or Speed Shoes help a lot too.
Equip FUEL TANKS ON ALL GEARS (except for Chu-Chu). Marlute's main attack is draining your fuel and using it to damage you, so if you block his main attack, he's basically dead. The only thing left is a powerful all-gear attack, but if you have Frame HPs ready it won't be that hard. Use either Z Chargers (if you're cautious) or GNRS 50 (to rush things up). Or both :P
Metatron has no outstanding skills, except for the fact that he can damage you a lot. It's mainly a question of turning on booster (or equipping speed shoes) and start attacking him right away (I had some good results using Emeralda's Ether, too). Just use Frame HPs when you have to, this boss is only here to waste your fuel and HP, so save your favorite gears for the last battle.
Sundel will choose one gear to counter-attack right away. If that gear attacks Sundel, he'll unleash an all-gear attack for over 5000 HP (and any other attack by the other 2 gears will result in Sundel using regular attacks against the chosen gear). You can guess that you must NOT attack with the chosen gear (charge, or cast some support spell on the allies if you can). This can be rather simple, but Sundel will quite often cast a healing spell for 8000 HP, so this is another battle that requires more patience than anything else.
If you destroyed all four orbs, this battle should be relatively easy. Equip Anti Angel Systems, Frame HPs and Z Chargers (I won't even mention Holy Pendants and Speed Shoes from now on :P). With this, Deus will barely damage you (including those irritating angels) with the exception of an attack that damages all gears for over 10000 HP. Don't worry about the next boss, you'll be fully healed beforehand, so use your best attacks (Infinity Level Deathblows) until Deus is down.
Urobolus will do barely 1000 damage with his regular attacks, plus his only powerful attack takes away 7000 HP (and isn't used often). You're alone, but Xenogears is fully healed, so the chances of you losing are slim. Just use regular X, Level 1 Deathblows (I didn't even bother with the Infinity Deathblows) to finally finish the game.