Call of Duty: Modern Warfare 2

For this you must Kill 4 enemies in a row while downed in Special Ops. You have to do it with another player, as if you do it on your own and you go down, you're already dead. We found that the one of the easiest and best places to do this is in the alleyway behind the cafe in the spec ops mission ‘Homeland Security’. Make sure your partner is close behind you, so enemies walk past you so you can get the kills when you are downed. However if you keep getting shot while you are down you will loose your pistol, so try to get half behind some cover when you are down, and force the enemies to come to you. Here is a video by Nextgentactics to help:

https://www.youtube.com/watch?v=pSPg-bOEb_U

There is a second but harder way to do this and this is by playing the spec ops mission 'Semper Fi', though it's still the easiest way to do it with two controllers on your own. At the start of the mission you want to line up your players so the guy going for the trophy is at the front, then get his controller and launch the predator missile into a group of four or more people. Now quickly before the missile hits, get the other controller and knife the guy. If you fail the first time, restart instead of keep trying as it gets harder and you'll get swarmed.

https://www.youtube.com/watch?v=jk2Di58jy2U

(Credit to MrBlingBlingNr1 for the video)