Outnumbering in Unknown
I read the rule as enemies (plural) being outnumbered leads to an enemy (singular) having it's damage reduced.  So each enemy has it damages reduced the same amount by being outnumbered.  I would calculate the reduction in damage as follow, with S being survivors and E enemies.

Damage reduction = (S-E)/2

So if a enemies did 3 damage and you wanted to reduce it to 0 you'd need to outnumber the the group by 6.