you need to make instance variables that contain the UID and pick the healthbar based on which enemy is hit.
Create Enemy
Enemy Spawn Healthbar
Healthbar.EnemyUID = Enemy.UID (make an instance variable in Healthbar called EnemyUID)
PlayerFire on collision with Enemy (when the player shoots the enemy
pick Healthbar where Healthbar.EnemyUID == Enemy.UID
set Healthbar width to self.width - PlayerFire.damage (or whatever)
then add
if Enemy.health < 0 then destroy