Beating the Nexus 51 Insurgents (Mall Raid Arena Mission)
Keep an eye out in the black market shop for a weapon called something like Experimental Dozer N. Its a shotgun/electrical weapon hybrid that helped a lot during that fight, and other fights too.
I think it does 40 dmg per shot comprised of both electrical and ranged damage with 16 armor piercing and a 20 round standard type magazine. Im pretty sure its fully auto, but its very controllable. Costs less than 10k I think. I remember being able to kill 5 to 6 enemies with one shot if they lined themselves up properly, but that might be due to a ranged skill specifically.
It can also kill Golem sized guys on this level specifically. I think it kills them in only three or four shots. It wont damage the Golems in full metal armor with the helmet, but they dont really show up until after this level.
Also, Holding control and pressing middle mouse button will tell your hirelings to hold their position and its usually a good idea to have them hold near the bottom right corner and focus mainly on watching for guys releasing the locks.
Explosive weapons also help clear guys off the package when they’re releasing the locks. Other than that try playing older arena levels on a higher difficulty to level your character.