After a campaign run through you should be on approx 300 kills (you can check your progress on the extras tab at the main menu).
The best place to farm kills is the boss fight with Johnny at the end of episode 6, just keep killing his minions and leave him alone. cheevo will pop on the 400th kill and can be done on your first run through, but going back and replaying the chapter after the campaign will make the grind easier as you will have more upgrades.
(p.s. I saw another guide saying to grind the final boss level, but these ghosts DO NOT count as actual kills).