Orcs Must Die!

Getting kills is the point of the game, so this will come naturally from traps, spells, guardians, and personal (bow/sword) kills. When I finished the main game, I had around 9,000 kills, so you should get this one well before finishing the game. There is no need to farm levels or do anything like that - just play!

Note that guardian kills (even those already present in some of the levels at the start) DO count. Getting kills and then losing a level DOES add to your total. Unfortunately I'm not sure about getting kills and then re-starting a level.