hmmm in the right side of construct2 there is a folder called "families" you can create a sub-folder and put all these enemies sprites there.(i don't know how because i don't have it)
and make a global variable that compared with the enemy health variable.(another event)
after that just in even make a condition if bullet or something...collision with the enemy it subtract from enemy health.
so just change the variables of the enemies and done...