Just do a: When hero gains a level Condition: Integer - Hero level of triggering unit equal to (the level you want it to evolve on, e.g. evolve on level 10 then set the integer/number to 10.) Action: Create 1 (unit) at position of triggering unit for owner of unit (triggering unit)
that's a very simple and basic one. For multiple evolutions for a single hero you would need dialogue triggers and I don't think you'd like that too much.