Try using blind or invisibility spells to your advantage.

As I am not playing no losses strategy and I did not want the battle to last for eternity I just put all I had :)
I fought with trolls, black knights, black dragons and T-rexs (had a lot of eggs).
Only trolls survived after about 55 turns of battle. Lots of turns were also put into fighting Friendly Gremlin towers, because there were 3 left after Gremlion died :)
(there was no more place for enemy units to appear so imagine - thats a LOT of enemies to kill)
Yet my trolls finished with 170 of attack :D

Hm, actually I recall beating Gremlion with zero loss as a mage and black dragons as my only army in a single stack, but I can't remember other details.

That is strange. Gremlion deals lots of physical damage and you can't enhance black dragon's resistance. They also cannot be healed or resurrected (without help of some other units).
There are also a lot of enemies summoned every turn.
I know that it is possible to beat him no losses with single stack, but I never found any trick for playing black dragons only...

Oh yes, I used demon portal a lot, that always saves the day in any combat, probably the best spell in the game.

But still if Gremlion retaliates to your dragons' attack - it deals about 1000 damage.
I think I will leave it the way I have. It took me a lot of time to finish this battle...
(waiting for all units to move and my computer got slow because there were too many of them)

LOL, the whole point of the mage-single-stack-black-dragon is that black dragons don't do anything except evading with their high speed, have high health and being immune to magic..they don't attack anything!

You'll need:

1. Paladins
2. Royal Griffins
3. Archmage
4. Phantom spell
5. (Magic Spring to replenish mana).

Combat sequence as follows:

1. Get a stoneskin on the Paladin
2. Move Paladins to attack Gremlion.
3. Put the archmage right behind the paladins (middle)
4. Move the Royal Griffins next to the Archmages, but with 1 cell free in between them (move them to the right side of the battle area) to allow Gremlion to put a tower down in between the Archmange and the Royal Griffins.
5. Use Archmages' arcane shield on paladin every time you get the chance. Otherwise use his trance mode.
6. Use Royal Griffins Heavenly Guard to get an extra ally. Do NOT use it to attack Gremlion. Use it to take down the towers in 1 turn.
7. Use Royal Griffins to kill the gremlin towers.
8. Phantom Paladins as the battle goes along to resurrect fallen troops (from Gemlion's Area of attack).

I used these three units and this strategy on impossible mage no loss. I had 35% physical resistance from items and only 25% magic resistance (Anti-magic diploma). All of these units have inherent magical resistance, so you don't need much more. Furthermore, I had even less intelligence/mana/mana reg than you had (4-6 mana reg per turn, 24 int, 100ish mana) and I beat him on turn 30-40 or later. Basically you just get the paladin/paladin phantoms/archmage to hit him and wear him down ever so slowly. To finish him off, remember to make sure all your units are rezzed and finish him off with the Griffins or whatever unit has a turn after the paladins in the last round.