Get 10 multi-kills in the Shanghai mission in the Campaign.(20 G)

This is attained during Mission 2: Shanghai. You can begin getting this achievement as soon as you exit out of the elevators. I recommend either using a shotgun, or another powerful weapon that you can get quick kills with in short range situations.

The key to this achievement is to use grenades to kill groups of enemies. Additionally, you can also throw a grenade at 1 enemy and then shoot and kill another enemy as the grenade explodes for it to count as a "multi-kill". It is also key to use claymores in the lobby area, and also on the helipad, which can easily get multiple kills for you.

After the helipad, you'll be running through some alleys as you try to avoid a tank. This is another good area to throw grenades and kill multiple enemies that will be grouped up in small areas.

Also, at the very end of the level, you will have to take care of two tanks, which will always result in multikills for you.