This achievement should, at most, take 1 or 2 minutes of your time. Simple. And as of currently, this is the shortest method I know of.
Start a new game from Nova Prospekt (considering the fact that you have gotten that far). From the beginning, in the 3rd or 4th cell on your left, there is a loose toilet just haplessly sitting there. Now, carry it with you down the stairs and to the hole that leads to the next area. Launch it at the head crab that jumps out of the hole, and your achievement will pop.