The Last Kids on Earth and the Staff of Doom

i found a great area to kill enemies fast, go to level 4.1, and all the way left is a warehouse. if you go inside around 20-30 enemies spawn quick.

if you die, you start right back at the warehouse

if you exit, and go back in, you respawn the enemies.

i found this area went very fast to work on kill achievements, for what seemed like a daunting task postgame.

there is even a character changer just outside the door.