I got it playing Eye of the Storm (last level of Sorcerer's Keep) using the strategy in this video:
http://youtu.be/K2e5d378WSU

It 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.

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Tcarr9's Tiny Heroes video walkthroughs are available on YouTube: http://www.youtube.com/user/Lordkalvanmidnight