First, head to the Floating Continent, then follow the map to the cave. You'll need The Invincible to access it.
Boss: Bahamut
The only problem with this boss is its Mega Flare attack. Either you have to survive the attack or you can kill him before he uses it.
Chests (13):
- Heavenly Wrath
- Earthen Drums
- Phoenix Down (x2)
- Arctic Wind
- Golem Staff
- White Musk
- Bacchus's Cider
- Chocobo's Wrath
- Turtle Shell
- Eurytos Bow
- Kotetsu
- Elixir