Oh I have never used pick highest but this would also work. I thought it was to do with z order :D
You can use pick highest attack stat and before this have a condition that the enemy stat is greater than 10, less than 21. This will pick the highest of the 'remaining' enemy instances.
In general I think the enemies should be different objects or you end up with a lot of animations on one object. If you want to pick all enemy objects together later you can use a Family of enemies.