Orcs Must Die!

This can be completed on any difficulty. If you're having trouble with War mage, try Apprentice. It'll give you skulls for upgrades, new traps (you can take all traps into earlier levels), and kills towards 30,000.

There are plenty of step-by-step guides to each level. If you find a good guide, please post a comment.

On the final level, try using the logs and some minions through the first couple waves, then set up your favorite kill zone with the money received. My favorite is hanging mace over tar with arrow traps on each side, but there's a lot of options. Be sure to equip a spell to deal with ogres. I recommend lightning.