Here's how I did it without losses on impossible as a mage like you.
Against K'Tahu:
- he is pure physical damage dealer. So stock up on units with high physical resistance. I did it with this lineup: Paladins, Knights, Horsemen, Guardsmen, Demonologists (for extra units).
- Put 2 units in front of him - Knights on his weak spot (in front of the unprotected hand) and Paladins if front of his armored hand. Paladins have a tad better defense than knights, and this is the reason for that.
- Put Stone skin on the Paladins and the Guardsmen. Then add stone skin to the Demonologists and Horsemen. Make sure that any unit that is not in front of his weak spot has stone skin on it to prevent huge losses.
- Now hack away! When he summons allies, beat them up with demonologists, horsemen and guardsmen. If you find it hard, have the unit that is in front of his armored hand to help.
THat should do it, generally
For Zilgadis:
Zilgadis inflicts physical damage when striking and magic with his mass attack. Make sure you have items to boost both. If not, it isn't so bad anyway.
I did it with the same lineup against K'Tahu.
1st round, mass haste and stone skin on the horsemen. The task is to get the horsemen to hit Zilgadis and suffer a counterattack, so that there is a unit in front of the boss. Otherwise there is a high chance he will do a mass attack 2 turns in a row.
2nd round, get the Knight on his left side (or right side) and the paladins on the opposite side. Cast 2x stone skin to both of them. Now hack away with both those units while the rest take care of any towers zilgadis summons.
Also, on both fights, save mana for resurrections after the boss is down and there is a stack of gorguanas vs K"tahu or any gremlin tower or a ranged stack vs. Zilgadis.