Do you mean each enemy has it's own health bar, for example over its own head?
If so, create a sprite called "healthbar" Add the pin behaviour to it. Add an instance variable called initialWidth and set it to the starting width you want for the healthbar.
Add it to a container with the enemy; when an enemy is created it will automatically create a healthbar object. Give the enemy "health" and "healthMax" instance variables - set both of these to the max health of the enemy.
Add this code:
On enemy created: Set healthbar position to (enemyX, enemyY-(enough pixels so that it appears above the enemy's head))
Pin healthbar to enemy.
When an enemy takes damage you want to reduce the width of the health bar, e.g.:
If enemy collides with bullet: enemy|subtract 1 from health
healthbar set width to healthbar.initialWidth*(enemy.health)/(enemy.healthMax)
Because the healthbar is in a container with the enemy, only the healthbar for that enemy will be affected.