I should explain this somewhere everyone can see it, but every bit of damage the monster takes adds to the armor, and the armor decays by a percentage every frame. If you bop a monster, the armor should be almost gone by the time you hit them again.
The actual damage taken formula:
damage taken = damage * (1 / (1 + armor / (maxHealth / 10)))
so if a monster takes a tenth of its health as damage, the next bit of damage it takes (if taken before any armor decay) will be reduced by half.