I ground out my 20 kills at the checkpoint after leaving the Hall of Heroes shortly after grabbing Shock Jockey. The chapter you want to reload is "Return to Hall of Heroes". (thanks perfectdozen for clarification) You can tear in a Mosquito, which is a floating automated machine gun, right in front of you in the air in the back of the area. This Mosquito isn't very tough, but it has a good line of sight to the enemies below. Between Slate's men and Comstock's men fighting, and them shooting at you, it should claim a few kills with each of its lives. I had two enemies with shotguns rush me on the balcony while I hid back and let the turret do its work.
You can help it along by putting a few shots into each men to wound them to make it easier for the turret to kill. If it gets destroyed, you can re-tear it back into existence a few seconds later while you hide on the balcony. Once you let it kill everyone in the area, restart checkpoint and you return immediately where you were outside the door for more boosting.
There are two allies before this one in the game, but the checkpoints are further away, or it will only help kill one enemy. This location only took me a few minutes.