"Health" has to be a family instance variable. Next, the simplest method would be to add "On created" trigger for each of your enemy.
Enemy 1 -> On created -> Set Enemy 1 "Health" variable to 10
Enemy 2 -> On created -> Set Enemy 2 "Health" variable to 20
...
The collision event will now have to look like this :
Enemies -> On collision with bullet -> Subtract 1 from "Health"