Killing 2000 enemies in total will take about 1.5 playthroughs of the game. After one playthrough your total killcount will be between 1300 and 1500. Just keep slaughtering away, untill you have 2000 kills, using the Replay Chapter option. You can check your killcount via the Pause menu and Statistics. This achievement will come while working on the Samurai achievement.
Note: if you start a new game, all statistics, skills, etc will be reset to zero, so choose Replay Chapter from the main menu to get your killcount to 2000. Don't start a new game.