By standing at 31782, -54745, 9156 in Isle of Dawn (-26,1) I am able to spawn what seems to be an infinite supply of mobs at roughly 32520, -55849, 9019). I only stood through thirty spawns, since by that point the rest of that mountaintop was respawning, but then spawned every thirty seconds or so and would be a source of infinite experience, at least up to the point where they no longer were xp.