Pretend that your enemy has the original point at his center.
Add an imagePoint below his head(could named it "neck")
Compare the Y of bullet when collide with the enemy:
If Y < Enemy.ImagePointY("neck") --> Instantly Kill
Else ---> Subtract 1 HP
OR you can have a fake head which pin to your enemy head but opacity =0, then you can check the collide with the fake head to kill him instantly.