Yeah I'm gunna go with a minigun too. Ideally a GAU-8 Avenger from off the A-10 modified to not need a plane to shoot from, probably mounted on a semi-trailer.
Regular minigun is pretty doomful, but anti-tank minigun is where the real class is at. The thing is that a regular minigun can't shoot through everything. Most things, but not everything. The avenger really benefits from being chambered in 30mm and exclusively firing high ex and armor piercing. It doesn't matter where any of them hide, or what the protect themselves with, they get to die.
Also, not to be picky, but wouldn't the zombies attack the criminals anyway ? Meaning that by the time they are done dicking around, it would just be zombies.
Even in that case, I stand by the Avenger. The only better weapon is some kind of orbital weapons satellite.