How to play a mage on Impossible difficulty?

[Sigma88]

KB:TL is the only king's bounty game i've played so try not to mix in tips from other games like spells to use and such.

As the topic says how exactly do you play a mage on impossible?
Right now I'm level 20, about half way through finishing elven lands, I just cleared Magic valley and thousand river and on my way to clearing great forest. My leadership right now is 12600+.
I've been fighting my way through here and i hardly used any damage inflicting spells. But maybe i should but i don't have a clue where to start. Do the spells even do alot of damage? I ask because I'm not convinced that it does. Or maybe I'm considering the wrong spells.
The enemies health range from 10000 to 14000+ and even with a maxed destroyer and 26 intelligence, according to my calculations, some of these spells are only doing a couple 2000-3000. I find myself using crowd control spells all the time to make my fights easier. But i feel like all that intelligence i have is a waste. However it does help with resurrecting my guys.

Can someone any tips on how to play a mage?