Having completed impossible as a mage, with a strategy of minimising losses at all stages, the best units I found for a mage were:
Levels 1-15
Royal snakes, inquisitors and any 3 ranged units
Offensive spells and ranged units decimate the enemy ranged units, and once the enemy only has melee left it's pretty easy. Inquisitors res at the end if required.
Levels 15-22ish
Sprites (eventually switched to dryads around when you get to elven lands), inquisitors, orc shamans, emerald dragons and elves
The sprites/dryads do pretty much as much damage as all the other troops combined (using the +3 to everything for female fighter ring, of course). With some careful use of wait to take advantage of their high initiative, timeback and resurrection, you won't lose any of them even if they are a bit fragile.
Orc shamans are great for all the reasons mentioned in this thread already.
Emerald dragons keep your mana stores up, and can pull enemy troops that are close to reaching your ranged troops away from them.
The elves are the least important unit in the combo - could have replaced with any ranged. Royal thorns would've been ideal but I didn't have any in my game and didn't find the +100% damage crown.
Levels 22-30
Once you get armageddon, all you need is one stack of dryads. They have a very good hps to leadership ratio (perhaps best in the game?), so they can survive 2 or 3 armageddons in the first 2 rounds of combat (then get timebacked and start afresh).
In addition, they have good initiative and speed, and plant thorns/lullaby are both rechargeable skills. Once the fight gets down to them only having slow-ish melee units, it doesn't matter how huge those stacks are, they will die eventually to your dryads because they simply can't hit them.
With this strategy I was able to clear every single end-game mob on impossible without loss.
Note: highly doubtful that this strategy will work as the other 2 classes since they can't launch 3 armageddons in the first 2 rounds!
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