Me and my friend made it to round 40 just the two of us with this strategy. We both got LMG's and pack a punched it till we got "turned" pack a punch. He had mule kick so his other gun was equipped with dead wire. So what we did was i would shoot the incoming zombies, one bullet at a time until I get a zombie to turn. What my friend would do is shoot that turned zombie until it's shocked with dead wire. This is OP so now the turned zombie is killing other zombies and shocking a shit ton of them too. This saves lots of ammo and lots of time, letting the turned zombies do all the work. I finished that game, 40 rounds, with only 800 kills lol. Other useful tips is to have "cache back" gum, the max ammo power up. This helps when you're low on ammo. Some other tips is to have "on the house" and buy 4 perks and then use on the house so you can get either dead shot da curry or stamin up. Hope you get dead shot da curry. Also, at the start of the new round, it's helpful to activate as many traps as you can, this also helps save time and ammo. Make sure you activate the trap near the power where the 2 doors/hallways are LAST so you don't block yourself in from entering the catwalk