Enemies: Bahamut lvl 4 x1 (Boss)
Dryad lvl 3 x2
Tin Soldier lvl 3 x5
Replacements: Bahamut with Grizzly Bear lvl 4 x1
The bad fairy Malevolenta has summoned a mighty Bahamut to block your way into the castle. If you don't keep yourself at full health, the Bahamut's attacks are easily capable of wiping you out, so try to rush it with all of your allies and overwhelm it in a single round.
Instead of using a brute force approach, you can also use the Red Force Line to create a red Force Field and shatter it, causing huge fire-based damage to everything within the area of the field. Although this won’t finish off the Bahamut, it should leave it weak enough that one solid attack will.
Since the stage is won when the Bahamut falls, ignore the Dryads and Tin Soldiers and just focus all your strength on defeating the boss. It might be a difficult fight, but it should also be a quick one. If you're having trouble winning, consider going back and redoing the Tepid Pool stage a few times to grab a quick level or two. That should give you the edge you need to win here.