Thanks goes to HellCreeper666
for finding this method.
Load up a fresh game of Lifeline, do your best to aim at the general head area of the zombie cluster and spray your gun away. Aim assist should be doing most of the work. Once you run out of ammo in your clip and you start to reload your gun, press start and quit out then load up another new game of Lifeline (don't select "Continue") and repeat over and over.
You can basically get the gist of the above from this video by HellCreeper:
This is the best method that requires no preparation, is very consistent and takes about ten minutes.