Don't forget that any "treasure" left by the dead creatures will eventually disappear if the chunk is loaded and no-one picks it up. Therefore, if you kill 1000 mobs automatically, but aren't there for what they leave behind, you might as well just kill the last few. Also, you will get no XP if you don't do the killing yourself. For this reason, what some players do is get monsters to fall far enough to make them one hit kills and kill them whenever they want. Spoils from 20 monsters after 20 hits with XP included is still pretty good. Have a look at Manuzza's monster spawner right near spawn (the big grey building to the NW).