I got it playing Eye of the Storm (last level of Sorcerer's Keep) using the strategy in this video:
http://youtu.be/K2e5d378WSUIt was the second or third time that I played that level that I got it. That's a great level for working on the achievements that require huge amounts of heroes killed, mana collected, etc.
Tcarr9's Tiny Heroes video walkthroughs are available on YouTube:
http://www.youtube.com/user/Lordkalvanmidnight