Actually, it's not that bad! An enhanced "Call Reinforcements" spell would work like this.
The idea of capturing slaves is great!
Right now, I'm playing a game as Sarah. She starts with the Crystal spell. I quickly found half of the Necromancer's set. Now, I'm strong against all melee units. I manage to trap one unit in a corner with crystals before the enemy reaches them. The AI is slow and doesn't attack them, so the attack goes perfectly! :smile44:
Added after 23 minutes
It's unclear why, but this tactic doesn't always work; sometimes, they attack the crystals.
Added after 8 minutes
I figured it out. This tactic works if the enemy unit doesn't split into several units, which is what happened in the previous four battles.