I just did this today, and it worked nicely. What you do is clear all the bosses and
trash till the gauntlet downstair. Now all you have to do is spread the group in
is upstair gathering up all the zombies. The trick here is that the achievement does
not have any range requirement, so all you have to do is when the aoe guy upstair
aoe down all zombies, the tank and 2 aoe at gauntlet charge into the gauntlet, get
as far as they can while killing all zombies on their way
everyone in group will receive the achievement