There seem to be more and more 3 hit monsters roaming the battlefield these days, and we should probably have a unit that counters this growing threat. I can think of a few possibilities........
A) Deathstrike: this one is simple, if a unit with deathstrike wounds an enemy unit, the wounded unit is killed (regardless of how many wounds it has). This seems quite powerful, so it might be better to give deathstrike a % chance of triggering (death strike 5 could have a 50% chance of killing a wounded enemy).
B) Poison: a bit like deathstrike, but instead of killing the wounded enemy, poison would halve the wounded enemy's strength (including bonuses). The strength reducing effect could last for the rest of the battle, or it could last until the start of the poisoning player's next turn.
C) Venom : much like the previous poison ability, but instead of halving a wounded enemy's strength, venom reduces the strength by a preset amount (a unit with venom 5 would reduce a wounded unit's strength by 5). If a unit's strength is reduced to below 5 by venom, it is killed.
I think I prefer poison/venom, mostly because deathstrike steps on ambush's toes a bit.
The spider is an obvious choice to receive deathstrike/poison/venom. But a demon with deathstrike could work. As could a serpent (and possibly medusa, scorpion, and green dragon) with poison/venom.