Orcs Must Die!

This is a story related achievement, and cannot be missed. You will get this achievement upon completion of the 18th mission called "Killing Fields" on any difficulty.

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, post a comment.