...Which is a little annoying at first, since early on you'll end up using a good deal of mana on traps which'll only finish off the last few units in each stack (instead of potentially killing many times more) - but once you get to around rank 2, you can rely on the natural probability of killing a stack for each of the two pre-laid traps, plus whatever traps you may lay afterwards, to get you to rank 3 ...
awesome medal IMO.