Add a variable to your zombie called "health" (or really anything that helps you know what it's for). Set it to a value (we'll use 100 in this example).
On arrow collides with zombie: call function "Arrow_hit_zombie" (Set parameter 0 to the zombie's UID, and parameter 1 to the arrow's UID)
On function "Arrow_hit_zombie"
--->zombie (choose by UID (function.param(0)) -> set health to clamp(self.health-50,0,100)
----> Arrow (choose by UID (function.param(1)) -> Destroy
Zombie health = 0 --> destroy/set death animation