Description: Kill 2 enemies with one grenade.
Note: All perk-based achievements can be obtained by re-loading the checkpoint and keeping progress. See video in description for details.
I did this in the first level during the castle sequence. I went in guns blazing until the two Commanders retreated into a small room with a table in the middle. They bunched together in the corner so they didn't have much room to escape, also note that if you throw a grenade onto an enemy he'll get slightly stunned.