Assassin's Creed III Remastered

This can be done during DNA Sequence 10 - Memory 3 (Battle of Monmouth). Right after the sequence starts you will be in control of a cannon and have to fight off some enemies. Aim slightly to the left and as far back as you can. Shoot into the approaching group of soldiers and you should get the 15 kills on your first shot. If you miss the shot you can either restart the checkpoint or wait for another group of soldiers.