First, head to the Floating Continent, then follow the map to the cave. You'll need The Invincible to access it.
Bestiary Checklist:
Boss: Bahamut
The only problem with this boss is its Mega Flare attack. Have your White Mage heal using Curaga and Cura at all times. It's just a matter of time as long as you can keep up with the damage.
Exit the cave after you defeat the boss.
Chests/Items:
- C-269: Heavenly Wrath - B1, bottom corner, left chest.
- C-270: Earthen Drums - B1, bottom corner, right chest.
- C-271: Phoenix Down - B1, top dead end, left chest.
- C-272: Arctic Wind - B1, top dead end, right chest.
- C-273: Golem Staff - B2, 2nd bridge.
- C-274: Bacchus's Cider - B2, 3rd bridge, left chest.
- C-275: Lamia Scale - B2, 3rd bridge, right chest.
- C-276: Turtle Shell - B2, 4th bridge, top chest.
- C-277: Chocobo's Wrath - B2, 4th bridge, bottom chest.
- C-278: Kotetsu - B3, before the bridge, right chest.
- C-279: Black Musk - B3, before the bridge, middle chest.
- C-280: Phoenix Down - B3, before the bridge, left chest.
- C-281: Elixir - B3, after the bridge.
- I-067: Bahamur - after defeating the boss.
Give Bahamur to your Evoker.