[Bozos of Bones]

I can usually complete level 6 this time through. If you can't beat, don't worry, you get a chance later in the game, when you're like level 15 or so. I don't know about that fire stuff, I just state the obvious. NEver really used any fore spells for water elementals and servants. But generally spellcasters are good here with various shields(Death Armor, FireShield Red and Blue, Circle of Bones, Circle of Blades...) and paralysis spells(helpless targets can be hit automatically... almost every time) like the best spell of all: Chromatic Orb.
Try and see if Banishment has any effect on the creatures, haven't tried that one yet.