The last boss basically comes down to the tank sticking with the boss, and the DDs immediately taking out any adds that burst through the first wall, as if they break down the second one, you'll be swamped and wipe before you even know what's happening. As long as you can keep the flunkies from breaking the second wall, you'll be fine.

As for the flan, you really need to keep it's parts close to the bomb in order to do damage to them/split them, as they're utterly invincible otherwise (well until they've been blown down into 8 separate parts ).