Do Haunt 21 Zombies Have Stats Reduced With Damage?
No you do not reduce their stats - the zombies are killed by a single successful Might attack against them.

The special zombie attack rules listed in the traitor's tome state:


  Defeating a Zombie with a weapon that requires a Might attack kills it. Dynamite can also kill zombies. Any other attack stuns a Zombie as normal.


Normally monsters cannot be killed in combat but are only stunned when they would take damage, so there are no standard rules for monsters taking damage.

However when a haunt specifically overrides the standard rules, only then can a monster take damage. This is the case for the Zombie Lord, but there is no specific rule listed for the individual zombies.