Host a private Horde session and invite some friends. Set the difficulty to casual and the map to River or Security.

River: Run to one of the 2 level houses and have 2 guys sniping (longshot and torque bow respawn in a corner every so often), one in the bottom, one out front and on the bridge with the mortar.

Tag some grenades on the side of the entrance to the barn/house thing and anywhere else you would like, and place a boomshield on the up-ramp to the upper level, but keep normal locust away or they'll kick it down.

Security-Tag and re-tag some 'nades on the entrance to the scorcher side security panel place so locust cannot disable the lasers, then go inside the other security side with the mulcher. hide in there protected by the lasers.

Tip- After each round on any map down the last enemy so you have 30 or so seconds to run and find ammo before he bleeds out.