u/MadeAnAccountYe

Just in case anyone else was looking for a location to get this achievement reliably!

Go to Bone Beach- west of the miner boss, where the mimic spawns. Weaken the mimic using whatever primary weapon you desire, then jump right in front of the second waterfall to the north and use your special while facing that waterfall. The grenades should bounce perfectly to land about right in front of the stone to the right of the entrance.

Hope this helps!