On Expert, with CPU players, stay in the safe room, pick a shotgun, then if you stand with the CPU players lined up in front of you, you can fire one round and it'll take down all three, then you should easily be able to heal 2 of the downed 3, rinse repeat. Takes all of about 10minutes max to get the full 50, if you're in a rush to get easy points that is, otherwise, just play it normally ;)