Max out your mana, and when your mana bar is flashing, use another mana item to shit your pants. You can verify whether a battle is a boss battle or not in 1 of 2 ways. If you cannot use summon items (Jesus, Mr. Slave, Mr. Hankey, and Mr. Kim), you're fighting a boss. The other way is to use the Examine function and see if one of your enemy's lines says "Boss."