This can be done solo.
Mutators: Super Reload (Optional: Infinite Ammo)
Horde Levels: Barriers Lv 8. & Horde Command Centre Lv 3.
Buy the command base at the top of the map
As soon as you get the money, buy the second command base down
This will give you access to the barriers in front of the gorgon pistol. Where the gorgon pistol is (in the little circle thing) is where you are going to camp
Focus on building up both barriers to the max and repairing them at the end of every wave
Build the two horde command centres, but save them for the later waves of each set
You don't need to build anything else, save your money for the 2 things outlined.
If you don't have the infinite ammo mutator, pick up a spare rifle at some point instead of your shotgun (I always use the lancer so I picked up a hammerburst)
If the barriers go down on the later waves, back up to the top of the map. Pick one of the two sides and hang around the back of the square cover. Hopefully by the times this happens you should have killed most enemies so pick them off as soon as they come through the gates. It many also be necessary to retreat here during a boss wave, particularly if you get the Brumak.
When you activate the horde command centres, activate the one nearer the bottom near the start of the wave (after everything's spawned for the first time) because you might not get chance to do it later, activate the second one nearer the end of the wave.
Avoid as much as possible coming out in front of the gorgon pistol camping spot. So long as you stay behind the barriers the enemy cannot spawn behind you. Come out and its possible for that to happen. Only come out to activate horde command centre and if there's only a couple of enemies left for the wave.
Be careful of the frag grenades. This is the no 1. thing that killed me aside from bosses.