The easiest way to get this one is to find one of the easier levels that isn't too short, which has a protected area. Usually you put a mana crystal in a protected area because the heroes can't get it. Add skeleton to your list of defenses, and just spam skeletons into the protected area while playing the level.
You might be able to get it using this solution to Fails from the Crypt too but that's chancier:
http://youtu.be/clYaian7rY0
Tcarr9's Tiny Heroes video walkthroughs are available on YouTube:
http://www.youtube.com/user/Lordkalvanmidnight
