I'm almost certain that this is not the fastest method, but it's the most consistent way I have found so far. This is best done on lower difficulties as we will be keeping a big enemy alive so he can kill his friends and (hopefully) not you.

Immediately after defeating Angie, you are able to retrieve a "Broken Slab" from a crypt between the Chalice and the Church. The slab is the key to the Beneviento Treasure back near Angie's house, so we'll be returning there. On the way back you'll want to stock up on healing supplies and pistol ammo, as well as taking the LEMI handgun if you don't have it on you already. Save your game and head back to Angie's house. Upon returning to the cemetary, you'll find it is now guarded by one big and very angry guy. If he lives long enough, he will summon weaker enemies to distract you.

What you'll want to do here is weaken the smaller enemies with the LEMI pistol as much as you can without killing them, so that the big guy can clean house and rack up the numbers for you.I haven't figured out the best way to get him to actually hit the little guys, because his swings seem to be incredibly inconsistent on where they hit.

After summoning a few waves of three enemies, he'll summon less each wave. If this happens and you're still a little ways off from the 20 kills needed, just load the save earlier and run back. He'll be right back to summoning three again.