2mins. 90 kills.
Follow the video (below). It will show you what to do. In summary all you need to do is enter chapter select and choose "The Sabateurs" its one of the last or the final checkpoint on chapter nine. (note this is only on the DLC (chapters of deceit)!
Stay behind one of the boxes and keep killing until the enemies stop spawning. Don't walk anywhere near the mounted gun otherwise the baddies will stop coming! Once the wave is over you should've gained at least 80-100 kills. If you want more you can simply play split screen and hide the 2nd controller in a corner. This will prevent the AI from robbing you of your precious kills.
I highly recommend the grenade launcher. Its great for knocking out a few kills and will help bump your time down so you can finish the wave faster. Its all down to personal preferance but i found the loadout described in the video to be the most effective (for me anyway).