Hmm, not sure yet.
I may be way off, but if I had this challenge, I'd test out different types of Variable options first if HP is a variable, if I was sure the code would work.
Variable options in Variable Editor for your variable:
Actor Real (for affecting individuals specifically)
Actor Integer
Global Integer (default variable setting for general actions that affect all)
etc.
Otherwise, instead of creator, I'd insert maybe clone.HP if that's possible. I'll have to think about this one. I avoided having Hit point bars on my zombies and bosses since I wanted less clutter. I know Streets of Rage 2 manages it usually in the top HUD bar, but it's not necessary to defeating the enemy, but a nice indicator of progress, especially against bosses.
Instead of Health Bars, also consider changing the color of the enemy, so when it reaches certain Health points, it changes color, from maybe neutral, to red. And then the final hit it expires. Just ideas, good luck!