Each class has a Vitality Die.
- Conjurer - d6
- Thief - d8
- Cleric - d10
- Warrior - d12
When they take damage, roll that Die and add the damage to the result. If the total roll is above the maximum you could roll on that die (12 on a d12 for example) you could do two potential things A) Reduce the die by one step. B) Go out of action - I think that totally sucks, but, its an option.. you could forgo the OofA roll and use the Surplus above the Die value. So. If your Vitality Die is 12 and you roll an 8 + 6 points of damage, you OofA roll would be 2.
The way armor works doesn't entirely mesh with this - it would need some work. Perhaps you use a low AP value, and just 'tick them off' ignoring hits? The same way the 'Sunder Shield' Warrior ability works? Perhaps you use them as damage reduction, reducing the modifier you roll? Who knows. More thought needed.
I'll revisit this at some stage this week - get something more solid.