Orcs Must Die!

I found the easiest place to do this was back in level one. I set out some Sticky Tar Traps in the narrow opening before my archers and continually set Boom Barrels in there and then shot them with my crowwbow. With this method I Got the achievement well before the final wave was complete.

This can first be done on the third level and is not to hard to do. Before you start the wave set up barrels with a big enough gap between them so that when one explodes it wont start a chain reaction. Now start the wave and shoot the barrels as the orcs walk by them.

Heres a youtube video if you still need help.

Alittle easier is that you should:
(As JORAX suggested this is level one)

1. Put two tar traps right in front of the door.
2. Place one bomb barrel in the middle of the large room.
3. Use wind belt right in front of the door to keep the orcs on the bridge beyond
4. When you run out of mana back up behind the bomb barrel.
5. As soon as you have mana again use it to push the bomb barrel towards the tar traps.
6. Rinse and Repeat.

I was able to get the achievement this way very fast and easy.
Some extra information if you are having problems with this achievement popping is that if you have the DLC then you need to delete it (you can download it again after you have the achievement).