There is another cheesy way. Since the enemy stacks are huge, all you need is one high-initiative unit, 1 tank with 30% phys resistance, gorguanas, target spell and ball of lightning maxed.
Turn 1. If you're playing Paladin or Warrior, use Target on the Tank straight away. Gorguanas cast Mark of Blood on the biggest stack and the pet summons a ball of lightning. With any luck, over 80% of the troop will be gone just like that. If you're a mage, target + stone skin on the first turn.
Turn 2. Cast Stone Skin on the tank. 1 or 2 charges left on the target spell. If you're a mage, flaming arrow on the biggest stack (so that the post effect can diminish their size) and lvl 1 phantom on the gorguanas.
Turn 3. If you have 2 charges left on the target spell, cast lvl 1 phantom on the gorguanas. If you have 1, you're hard pressed to recast it and slap another ball of lightning. As a mage, use mark of blood on another big stack and use ball of lightning again. Spells - target if needed and another flaming arrow to get another troop burning.
At this point, all you need to do is rinse and repeat
Ball of Lightning + Mark of Blood from the Gorguanas make the big stacks die faster than lvl 1 units.