Fairly self explanatory. Just don't turn on the power. You can either just hold out with 3 friends or find a good place to run a train of zombies. You can use the mystery box as that does not require power.
A good method is to buy the R870 Shotgun from the wall as it is a OHK up to around round 8 if you hit them directly in the chest or head AND build the Sliquifier (See Slippery When Undead). You can find a decent spot to run around in circles then just shoot a zombie. If you learn how to use the gun correctly you can kill 10+ zombies with one shot. Picking up the Perma-Perk Juggernaut works well too making you go down in 4 hits instead of 2. 6 if you buy Juggernog.
Another good strategy is to make your way towards the Galva Knuckles. They cost 6,000 posts but are a one hit kill until round 14 for both Zombies and Nova Crawlers. This means you should be able to get a free perk if you melee them all.