Here is my video that goes through the whole process. I'm testing out whether crouching a little more to the right, behind the taller cover, works better. Specifically so I don't have to see enemies at all and not worry about bodies building up. And as well you don't have to do what's in the video, this is only for racking up XP while you're away or asleep. If you want you can sit there all day and get headshots at 10 xp a piece. When your teammate makes the kill it's 5 xp, and he does it slower than you would.
Make sure you are in Assault mode and not Infiltrate. You can do this by pressing the Back button. And make sure you're silencer is not attached. This will cause your teammate to have his silencer on and it takes longer for him to kill the respawning enemies.
If you're still having trouble go to the above mentioned thread, lots of people have posted some great tips.
Thanks goes out to MetalHeadZach for finding and posting this glitch. This will save us all a lot of time.
You're right mi_man_man. I've been testing this. I thought you had see the enemy every time it spawned for your teammate to be alerted to the enemies presence. But this is not so. I've been crouching more to the right, behind the taller cover, and not looking at the enemy at all. This prevents him from seeing and shooting you. It also prevents the bodies from piling up which will slow down your xbox to a crawl.