Put the difficulty on anything you wish, put infinite ammo on, start a match and hope that by the end of it someone joins. It is easier to do this if you make your own room, where you can get a lot of the kills before anyone else joins.
If no one is in your match when you finish the level, it does not count as a win.
The best thing to do in this situation is to wait at the very end of the level, and as soon as someone else joins your game, finish the level. This will count as a win and you wont have to wait for them to get to the end.