Attacks: Giga Breath, Drain Bark, Poison Ball
This is a tough boss, so make sure you have everyone standing in the shape of a square. Karin in the back, your strongest golem next to her, Raigar and Astuma in the front. Golem, Atsuma, and Raigar attack (Raiger also needs to cast shield on the party), have Karin heal the person next to her, herself, and the person in front of her (however, if Amazonia hasn't caused too much trouble feel free to let her attack too if she's within range.) Make sure you use Atsuma's Summon Phoenix whenever you can.
It may take a couple of attempts, but you'll eventually be able to defeat him for the achievement.