I accidentally discovered a trick to defeating the Plague (the boss that casts doom on all your party members). Kill one of your characters and then revive them. As soon as you revive them, Plague will cast doom on everybody again and the doom counter will go back to 10 for each person.

As for Bahamut, you can win the battle by continuously casting wall on Rosa. Everytime Bahamut casts MegaFlare, the spell will be reflected. Be careful though, Wall can only withstand two rounds of reflecting MegaFlare.