Also, let me clarify something. If I understand what you wrote correctly, I think you might be making critical hits more powerful than they are supposed to be if you're taking 14+ damage from a single zombie attack. A zombie at maximum can muster 10 damage with a critical hit.
Are you adding the zombie's dice together, maybe? If so, that is not how it's intended to be played. You take the highest die, not the sum, to compute the damage.
If there's no critical hit, then you subtract the highest die from the losing side to mitigate the damage.
If it is a crit, there are no dice on the losing side to mitigate the damage, so you subtract zero. The only other thing in play is that a crit allows both of the highest dice to apply their boons. Since zombies have boons on both of their attack dice, it would apply both.
Thus, a zombie which rolls a best-case critical hit of 8:8 would do 10 damage (8 for the highest die, minus zero for the crit, plus 1 for the Ragged Claws boon and 1 more for the Relentless boon). I don't see how it could muster 14+ damage unless you're referring to multiple attacks.