I'm also slowly progressing in my no loss impossible mage game. I use a lot of the tips and strategies players (DGDobrev, TemjinGold, Zhuangzi, Elwin, The Rider, Zechnophobe and others) posted.
My pet dragon is my main weapon and leveling him up fast is very important. His xp per fight is based on (a) the base xp of the abilities he uses in the first 10 turns, and (b) the strength of the opposing army (from Very weak to Invincible).
I try to optimize the (b) part of the equation by changing the number of units in my stacks. I split 2 of my stacks and put those 2 stacks on reserve. I twiddle with the number of units on reserve until the enemy's rating goes from Lethal to Invincible.
Of course, should I lose the fight on Invincible, I reload and try it on Lethal, or even lower
. This way, the pet dragon gets the best xp possible.