Depends on how you have things setup on your Event Sheet(s). Either way, you might be able to make use of a "For Each" Event, which you could use to remove a set amount from a variable for each instance of that object/sprite taking damage(within the range of effect).
Or like I said earlier, create an AoE sprite(visible or not). Then if the enemies that come within that range and collide with it when it's active, then have it remove a set amount off a variable for those objects that collide with the AoE sprite?
EDIT: There's always a handful of ways of doing any set effect within any game engine. It all depends on how you currently have things setup for the game, and what makes the most sense to use, to edit the variables you have set for enemies' hp value.