Abyss Hallway
To simplify the explanation just a little, I have incorporated a map system.

This is a map of the next section. The path to the boss is marked in yellow. Follow that path if you want to just beat the boss. To get all of the extra items along the way, take the lavender path. The gray areas are hidden rooms. You must strike the wall with your whip to access them. The red dots are Heart Max upgrades. The orange items are Health Max upgrades. The light blue items are Magic Max upgrades. You can only get the upgrades that have lavender lines going to them. The other ones you will need more Magic Items to get to. The red squares are save points. Always save your game at a save point. Not only can you save your game but you can also replenish 100% of your health and magic. Be aware that I will not tell you to level up. It will be your responsibility to do so. I will also not remind you to go back and grab any extra items that you may have missed. You can stray from this guide at any time to get extra items you missed.

In this first room are two extra items. Grab these and head up to the next room. This next room has an extra item, but you cannot get it until you have the Roc Wing. Save at the save room and go to the next room. This next room has one extra item. Grab it and head to the next room. Head through this room to the next one. In here, grab the extra item and head to the next room. Save your game, grab the extra item, and head to the next room. There are two extra items in this room. Grab them both and continue on to the next room. This room has one extra item. To get it you must hold down and the attack button at the same time to spin your whip and take out the floor. Grab the item and head to the next room. In here, equip you Mercury and Salamander Cards. Ice whip the Brain Floats and use them as platforms to hop your way to the extra item in this room. Now, head down to the next room. Save your game here and continue to the next room. In here, grab the extra item and head right to the next room. Grab the extra item from the secret room and head on to the next room. Grab another extra item and continue to the next room. Head over to the boss room and whoop his ass. Grab the Tackle from the next room. Now, leave the boss romm and down to the very bottom of the next room. Head left to the next room and use Tackle to break the stone blocks. Head through here to the next room. In this room, head to the top and to the left to the next room. In here, take the third path from the bottom to reach Triumph Hallway.