Load up chapter 4 on easy. Proceed forward until you reach the first group of enemies. There should be a large tombstone shaped object to your left, climb up it and then face away for the enemies and hit O. You should now be hanging from it while facing your 3 attackers. Fire your pistol at them until you kill all three and then pause and reload checkpoint until you get this.
Also you should pause and save game after getting this so that the game keeps the medal points.