To get this achievement you can either play with some buddies, which makes it easier, or do it alone. I was able to do it alone.
Start up beast with the following settings:
Map: Checkout (you want a small one)
You can go for this achievement as well:
For the first 5 waves use the Savage Drone. This is just like your regular character with a retro-lancer. It's easy to get around and you can shoot at a distance.
Once you have gotten high enough you will get the Berserker. She's a beauty! She is the one a the bottom left corner and she'll cost you around $5,000 (you'll make that every round, EASY). Once you have the berserker unlocked just Run through everything (hold A) and then start pounding on people (RT). The berserker goes through every fortification so it's pretty easy to just run around and kill people.
On the final level your going to have to kill 16 people. You need to watch out for the guy with the "One-Shot" and the flamethrowers. If you see them take them out ASAP. If you see Prescott try to take him out. Once done the other COGs will somewhat rush you instead of hiding. This last level took me a few tries but just keep at it and you'll get it.
Some tips when your doing this are:
3) Try to get multi-kills. It'll give you more time.
2) If you down someone and no one is around then just wait till someone tries to help and then kill them two.
Hope this helps and happy killing COGs