Just got this while playing in multiplayer. Much easier, I suppose.
Select a game of Horde Domination, choose the Shadow tier.
When in game, pickup the bags and when you finally have one gas granade, hold on it. As soon as you have control of a zone, wait for the incoming horde.
As the zombies approach the point you are defending, stay close to your partners, throw the grenade and wait. Players will likely to pass in the gas while zombies are swarming around.
You'll get the achievement in a flash, probably.
A little luck is needed.