Dark Cloud

For this fight, I used Ungaga and Toan. You must use Ungaga's swirl attack to blow away the black smoke of the boss and reveal the coffin then quickly switch to Toan and have him hit the boss since he should have a much stronger weapon at this point.

The swirl attack eats up Ungaga's weapon very quickly, so be sure to have something beside the one he starts with. If you 100% the village, the chief will give him Mirage, which is a much stronger weapon than the one he starts with. Auto-repair powder would be nice but not strictly necessary - just make sure you have a way to repair Ungaga's weapon as the fight proceeds.

Alternately, you can use Ungaga for the whole fight; all he has to do is stand there and twirl. Auto-repair powders in your active items box are necessary to prevent breaking his stance.