Okay:
Make a collision event in your hero object, and make it a collision with the enemy.
In there, make an if statement. (Control tab in the object window, it's on the right, then right-click the one that looks like a question mark. In there say:
self.sprite_index=Spr_atl_left
Click okay, or confirm, or whatever, then make a destroy event, and apply it to other.
Do this again, in the same event, but this time the question mark says:
self.sprite_index=Spr_attack_right