This is easy to earn. Once you get to the end of the game you will unlock the final power ability called "God Key" which gives you the ability to stop incoming bullets and throw them back at the enemy. Just equip the ability by pressing left D-pad button then hold down the LT button to stop and collect incoming bullets. Aim at the enemy and release LT when the power bar on the bottom left corner of the screen is full and your character's left hand is glowing to throw the collected ammunition back at them. Repeat until you've killed 50 enemies this way. Can easily be obtained during endgame by using it during endgame missions and while searching the maps for collectables you've missed.