Most consistent way to whittle down this acheivement is one, play on village vanguard, two play any class on waves 1 through 6, three when on the 7th/last wave change to pyro. Chase the tank that goes near the pit and air blast as many robots into the pit as you can.
Take uber canteens, focus on survivability ammo, dammage resistance, movement speed and health regen.
Use stock flamethrower.
This will take many, many playthrus of this map. But it will consistently make progress.