Slayer of Harpies

Reach, enforce (This creature's power and
thoughness can't be reduced and it can't be destroyed
by lethal damage unless lethal damage dealt by a
single source is marked on it.)
Whenever Slayer of Harpies survives combat
damage, you may have it deal damage equal to its
power to target creature with flying you don't
control. If you don't, put a +1/+1 counter on it.