It's fairly simple to get this. One way is go into Survival mode, and do the campaign mission "Last Stand" Once someone throws the switch in the basement, a whole bunch of zombies will start coming. Go onto the roof of the building, then climb antoher ladder to get you to the highest point. There will be a bunch of molotovs. When you see a pack of zombies, throw 1 molotov, then pick up another one. Rinse and repeat. Redo this level about 3-6 times doing this and you'll get the acheivment.