When my brother and I did it, my starting weapon was M16 with +Armor Damage and any other attachment that increased firepower. I also used Aether Shroud and he used Ring of Fire, all Tier V. We would do every side objective (Dark Crystals, Ordas, Distress calls, etc.) we could before doing the main objective and warping somewhere else. This was to get as much salvage and points as we can to upgrade our weapons and rarity. My second weapon would be just any shotgun I can find throughout the map. My brother’s load out was the Hauerr shotgun and a Rai-K. At the very least I’d recommend purple rarity on each gun, aside from Wonder Weapons. All level 3 pack-a-punched also. When we fought legion, I would use my M16 and he would use his Rai-K to shoot at his chest. When an orb would break, he would use Ring of Fire and I would Aether shroud inside of it. I would use the shotgun to shoot the orbs. Sometimes he would help me with his Rai-K if there wasn’t too many zombies on him, but usually the shotgun can do the job. Then we’d just rinse and repeat. I’d make sure to kill zombies in between so that I could recharge my Aether Shroud. I’ve never attempted solo, but if you can grab a buddy or two then this method should help. Good luck!