Just before finishing the game, I tried it and got a nice critical : 233K with thorns warriors.
To get that I maxed out the attack stat bonus with the Frenzy skill, killing all stacks with the thorns then letting the opposing dryads summons more thorns stack to kill, in order to have at least an attack bonus of 90 over the enemy defense.
For the damage stats I used the thorn crown, the whip and the poison dagger along with hell Breath. All of that boosted the thorn warrior damage from 1-3 to 6-12. A 400% boost.
And finally I used as a victim another thorn unit. They have a -100% fire vulnerability. So each point of fire damage is doubled. That mean each thorn have a base damage of 17. I tried increasing the vulnerability of the targeted unit with grease but it didn't worked. I guess the maximum vulnerability is 100%.
Finally the only necessary spell were doom (100% chance of critical) and Hell Breath.
With a stack of 2885 thorns it means that each thorn did about 81 damage. Not bad for a 8 leadership unit
A warrior with more leadership (I had 23080 as mage) should easily got more than 275K.
