Even if you're close to the end of the game and only have a fraction of the kills needed here, don't fear! You can complete the entirety of this achievement in the front yard of the mansion when you get to the lighthouse where Paulie is holed up. Once you've knocked out all the lights and made your way up to the front of the mansion, simply run from the front to the left hand side over and over using your darkness powers as needed to complete the percentages. The number of enemies spawned is endless, so you can just keep killing until you get the needed kills (if you've earned the Legendary Summoner achievement by this time, the needed Darklings should already be at 100%). Just be sure you don't go through the mansion doors, or the fight is over.
Granted, only two to three enemies spawn at once on each side, so it can be a rather monotonous endeavor if you have low percentages to start with, but it's an easy way to get the highest point total achievement in the game.