The easiest way, would be to have an invisible sprite behind some cover object, or multiple sprites. Then have an Event that checks to see if an enemy's health variable is below a set value. If so, have it find a path to that invisible move to sprite, and then move to that sprite. If you have multiple move to objects/sprites, you could have a choose action, so it chooses one of the sprites to move to.
I hope this makes sense. If not, I could go into more detail, or just make a quick example if I have some spare time.