Fire in the Yaung-hole!

This achievement can be done with more than one [Highly Explosive Yaungol Oil] you got a 3 sec window to throw a second or third bomb and you can still get the achievement.

You can attempt this achievement during this daily quest for the Shado-Pan.

The easiest way to get this is to keybind the oil, and wait around at the base of the hill, just south of the blockade where the mobs are clashing. Sooner or later a large number of mantid will show up, easily around 25+ NPCs altogether. Bomb the largest clump once, and quickly spam click to bomb the same spot again.

I was a bit confused while trying for this achievement, because I thought I was "killing" way more than 15 mobs. In reality, half of each group seems to live through every bombing. One of the below comments was correct, there is a grace period with the oil. Just hit the same spot two times in a row and you'll score the achievement.