How do I can damage only one enemy at the same position?

In my game, multiple enemies can be in the same coordinates. When i shooting towards enemies, I would like to substract damage to only one enemy. But how to do this if all the enemies are in the same coordinates?

I tried to do something like this but no result

I think I solved this once by picking the topmost based on Z-Order. IE. If two enemies are going to be hit at the same time via a collision detection, then it will take the one at the top of the Z order on the layer. I also did this with mouse click to loot an item that happened to spawn very close to another piece of loot.

Here is an example based on mouse click and having two loot items on the same coordinates.