Army of Two

You can do this in China on single player fairly easy. Equip yourself with the M134. Exchange weapons at the beginning with the AI ,run up the hill get behind the tree and let him do all the fireing and when you get to overkill run down and kill the enemies. After blowing up the bridge repeat in reverse. Get to you hovercraft rinse and repeat.
This will also get you pretty close to the $1,000,000 achievement.

I did this in about 25 minutes during the Kiev Mission (Free Veteran DLC pack) with the AI. I gave my partner a pimped shielded AK47 and ordered him to advance full power (press UP on your directional pad twice), As soon as he kills the first enemy, you will have the "Overkill" mode available. Press the "A" button and kill as many enemies as possible using guns or close hand attacks. Repeat this strategy during the rest of the mission until Salen and Rios crawl through an air vent (3rd checkpoint). Now quit, rinse and repeat.

For this achievement you need to kill a total of 75 enemies while in stealth mode of overkill. You can do this in single player and multiplayer but it is easier to do this in multiplayer mode. Just have the other player use a big pimped gun so get all of the aggro then it will ask you to go into overkill mode, enable overkill and you should go partially transparent go kill as many enemies as you can. Once you do it enough to get 75 kills you will unlock this achievement.

This achievement is earned by killing 75 enemies while in the stealth mode of Overkill.

I had trouble using the China mission, so I found a pretty good place to do with yourself and a second controller on Aircraft Carrier once you enter the ship. Before you start the mission, make sure you have a pimped M134 (the big machine gun). Once you disable the planes on the deck of the ship and make your way inside, you'll spawn in a hall with a riot shield in front of you and a door to your left. Swap your weapons with your second controller and have your main controller stand on one side of the door. Now, use your second controller and fire the machine gun outside and kill at least one person to fill your aggro.

As soon as you can enter overkill, grab your main controller and go into overkill, then run outside and headshot as many enemies as you can. Once you run out, just restart the checkpoint. Took me about 10-15 minutes doing it this way. This is also an excellent place to get the Co-op riot shield kills as well, and the 100 kills in the other mode of Overkill.