How do you beat the First boss in the diablo section? He spits blue fire and bats and seems to always kill me, any suggestions?

There are some stones in the room you can use for cover. Get behind them so the fire can't hurt you and start attacking with combos. If the boss is near enough, you will constantly hurt him and kill the bats as well.

I find this section to be easier with Clink, as his combo has more range, and if you're doing it right then you will be kiting, and won't get surrounded, and won't need Kaerith's aoe.