1) Don't conjure huge pets and then just run in and stand there or missile hoping the mobs will do all the work.

The successful conjurer will be fleeing a LOT keeping the servitors in the fight but themselves out of it. Use the elemental to open the fight, or initiate and the flee and come back when they are on the angel or the elemental.

2) Open with a really nasty scroll or wand. (wither, double energy drain, blind poison plague)

3) Also in general it's a very bad idea to have your familiar fight with you. Iggy starts bashing your familiar and it's done.

4) Against Kostyan and Igbah I would be using an archon all the way. One flee and a couple of steps and you are up to 100% health again.

5) If you just need to buy some time before you really start fighting have your air elemental lift you up, you will avoid all melee attacks.

6) Use your powers to find the big enemies at their weakest and then jump them.