This, along with All-Rounder, is one of the two big time-consuming, grinding achievements. You have to kill 2000 monsters across all playthroughs; note you do not have to save the game, as all kills will stack. If you really want to grind this out in one location, a good place to do it is at the entrance to the Water Temple, as there are at least half a dozen enemies at the entrance, and you have your entire crew to help. Another good spot to do this is at the end boss, as she will periodically spawn anywhere from 3-10 enemies. Just don't kill her and wait for her to spawn her henchmen, then kill them.
To give you an idea of how long it will take, I had around 800 kills after doing all the other achievements through 1 1/2 playthroughs. Took about 8-12 hours to grind the rest out.