Boss Strategies for Lunar Underground

Odin
Have Rydia cast Bolt 3.
Have Rosa cast Haste or Holy.
Have Cain jump.
Have Edge fight.
Have Cecil attack.
He should be dead in 2 rounds from the Bolt 3 spells.

Lunasaurus-Ribbons (2)
Have Rydia summon Bahamut all the time.
Have Rosa cast Cure 4 all the time unless you don't need it, then cast Holy.
Have Cain jump all the time.
Have Edge dart Hellwinds all the time.
Have Cecil attack all the time.
If someone dies, don't revive them. Keep bashing the Lunasauruses one at a time.

Plague-Holy Lance
There's a 10 second time limit.
Have Rydia summon Bahamut.
Have Rosa cast Holy.
Have Cain jump.
Have Edge dart Hellwinds.
Have Cecil attack.
Plague may cast Haste on you, but don't worry about it. It doesn't make your time go faster nor will slow decrease it. It only has effect on how fast you can go, so Plague is actually doing you a favor.

Tidarthian-Masamune
Have Rydia summon Bahamut.
Have Rosa cast Cure 4 on everyone or Holy if you don't need it.
Have Cain jump.
Have Edge dart Hellwind.
Have Cecil attack.
Don't bother casting Bolt 3, Raijin, or summoning Ramuh since Bahamut and Hellwinds to more damage.

Dark Bahamut-Ragnorock
Have Rydia cast Bahamut.
Have Rosa cast Reflect or Cure 4.
Have Cain jump.
Have Edge dart Hellwinds.
Have Cecil attack.
Have everyone use Moon/Light Curtains during battle to cast reflect on everyone so his Mega Flare will hit him instead. Don't cast regular spells. If you need Light curtains, you can steal some from Phase in the second part of the moon (crystal tiles).