How do I differ multiple enemies?

Hmmm... I think I may know what you are asking, is the AoE a circle? Either way, you could create a sprite set to the size of the AoE(area of effect), and set it be invisible on layer start. Then have whatever object causes the damage to spawn that invisible object. Then you would just need to check for collissions between the objects that get damaged, and the object/sprite you created for the AoE. I hope this makes sense... You could set the object to visible for bug testing, just to make sure the collisions are working, and the damage is being dealt appropriately during runtime, then switch it back to invisible, so the player does not see it. Unless you want them to see it, like a big red circle(adjust opacity as desired), that fades away over time or something on the ground, while the enemies/whatever takes damage.

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.

If your enemies are separate objects, you can use Families to associate them together.

https://www.construct.net/en/make-games/manuals/construct-3/project-primitives/objects/families

Good to know. I'm still fairly new to Construct. I've prototyped a bunch of different games, but haven't come across using "Families" yet. Reading through the document you linked to now. Thanks for the share!

No they don't share data. You create a family instance variable for the HP that each type will use, and another for the damage HP. Then back in the editor, you set the type HP for each enemy. Copies from then on will copy the same unique value. Your calculation then reduces the current HP by the type HP value.