Containers are your friends.
Put the health bar in a container with the enemy. When an enemy is spawned it will have it's own health bar spawned with it, and any time you pick that enemy it will automatically pick it's health bar.
Just do a for each and loop through your enemies. Each time it picks an enemy, it will also pick that enemy's health bar so that you're only updating just that one.
Here is a capx (R120.2) that demonstrates what I am talking about.