[DGDobrev]

Now you see what I was up against. 115 necros, 115 Demonologists, 338 druids, 187 Archmagi and some Polar Bears.

Here's how I did it.

Put on as much physical + magical resistance gear (Antimage diplom from Elon is very useful here - +25% magic def) as possible, as well as +defense gear in advance. Make sure you use high-hp troops that have some physical resistance (preferably 20% or more). Start by casting Target + Stone Skin on your paladins. Your paladins are the troops that must survive at all costs. He'll do geyser - geyser does physical damage. You'll suffer big time, but your paladins stack will be relatively safe - probably with a few troops lost only, as stone skin lvl 3 boosts the phys resistance seriously. On the next move, put stone skin on the 1 troop who has the least physical resistance. With the second cast, slap something powerful right in their midst, while your troops are charging at the units they can kill without suffering much.

If you manage to get the AI scared at this point, it will summon animals to bolster their ranks. If you get to that point, you're relatively safe. If not, keep you paladins in shape while he geysers and summon phantom paladins to heal your other troops. Extra mana can be provided by using mana accelerator max lvl on every turn. You won't have rage problems in this fight, I can assure you.

Eventually, you'll beat the crap out of him. I know he's hard - imagine me doing him without losses, but everything is possible as long as you experiment and know the game mechanics.

To go in-depth, here's that "troops you can kill" means:

Necros do phys+magic damage. Attack them with a unit that has some of both resistances
Archmagi do magic damage. Attack them with a unit with high magic resistance, like the paladins.
Demonologists to phys+magic+fire. Knights will be a good unit to tackle them.
Druids do phys + magic. Have whatever you have attack them.
The extra unit is usually a small animal stack and is irrelevant.