This achievement is a lot easier solo!!! After numerous tries in different gaming sessions and searching for randoms, I had only reached Wave 28. After following this strategy, I made it to Wave 56 ALONE!!!
There are fewer enemies, and they have less health depending on how few of players you have. Also you don't have to share scrap and can drop a turret from the start of the first wave in survival.
Run a Hamer MkIV with 3 Muerte Fiesta Numero 6s. You get this shotgun for Wave 5 loot on Settlement Survival. Also, equip Speedy Biped Legs. You are going to run a lot in the first 15-20 Waves.
From the start of Wave 1 you can immediately place a sniper turret. The best places to put your turrets is on top of the vegetation on the level. Spam sniper turrets whenever you have enough scrap. I never upgraded turrets until after wave 20. When you do upgrade your turrets, upgrade the back defenses first to ensure your scrap isn't wasted on a turret that might be destroyed by a burst transmitter.
This video helped me. It shows the angles needed to get turrets on top of mushrooms/trees out of reach of knobs. It will also help with locations of enemy spawns, however some of the spawns are randomized, but not major enemies such as Volt Droppers and Big Berthas. This guy also did it without support turrets. I dropped a few collection pods and dampeners after I felt I had enough snipers to limit my running.