I think you're underestimating how much it takes for 3xL1 to dish out 22 damage.
Supposing you remove the fortitude bit and that 3-of-4 party attacks hit:
Round 1: all three hit; fighter + rogue (with SA) take down 1 zombie, Wz hits another for half its health. Ten zombies get to attack your party, say 1/3 hit for 15 damage to party. Someone's down. But let's ignore that for a second.
Round 2: two hit; rogue (with SA) takes down Z2, Wz hits another for half. Nine zombies attack, 13 damage to party. Someone's down again.