The best place that I've found, AND the most reliable is The Great Crystal II. When you get in there, take Saggitarius Gate two. Go right, and you'll have a neverending spawn of Necrofiends or whatever. They kill for 2500 xp, and you can create major chains of them, because the spawn is random. Plus, Once you get a chain of 256 of them, you can (SPOILER)try to kill the Larva Eater that spawns at the nearest waystone and get the Danjuro, the most powerful dagger in the game.