There are two GREAT ways to get this, and a few longer ways. My first suggestion is:
No Mercy Finale: Play on expert, and kill your teammates. Then switch it back to easy and clear the hallway above. Now go back to the saferoom, stand a foot or so away from the door, and melee. You can also use your play and charge kit + a rubberband and work on this overnight.
Survival Mode: You can download survival mode free off of the marketplace. For those who don't know what survival mode is a mode where endless zombies attack you. So start up a game with bots and play. Since you are playing with bots, you won't last a long time, so use all your healthpacks and molotovs/pipe bombs early to take out the zombies! You can probably get a few hundred every ten minutes or so. This can also be done with friends online- but if you are going for survival mode time don't use all your equipment right away ;).
Third Suggestion(Mediocre one, the first two were the best):
Play the finales over and over. Each finale only last fifteen minutes or so, usually less, and you can easily net around 100 kills in this time. Just play like you normally would, except die before going in the rescue vehicle. Stay out and fight, so you can net extra kills then restart the levelw ithout going back to the menu. The best levels for this would be Dead Air or No Mercy.