Midnight Fight Express

I have found that a good mission to farm/accumulate kills is Mission 18 "Train Station".

Just throw objects at the crawling enemies called "the possessed" with and they should be sprawled out on the floor, then kick them once or twice with until the finisher prompt appears. For me, I was getting roughly 2-3% naturally per run while trying to do other achievements.

Master AO also suggests doing a Light then Heavy combo by tapping then holding to sweep the leg (Johnny) then you can do a Ground Finisher with .

With all the abilities in the skill tree unlocked, grabble an enemy with then press to choke slam them to the ground, you should now be able to finish them with