I'll normally use the Petrifier, the Porter's Ray Gun Mark II, and the upgraded AN-94. What I do after round 25 or 30 is start off each horde in front of the gunsmith. When it looks like most of then have spawned, I'll head towards the saloon, but go to the right side of it. I'll wait for the zombies to catch up, and then I'll walk around the patio of the saloon. Wait for the zombies to catch up, walk back to my original spot, kill the zombies, stand in the green mist while the zombies respawn, rinse and repeat. With this strategy I rarely even get hit, and it's also pretty fun to do.