A few episodes into the game and you will start encountering Oozes(the humanoid-type enemies) that have triangular shaped sides on their heads. These enemies shoot bone-hooks at you from a distance. These are the "bullets" that you have to deflect with your knife. All you have to do is give yourself some distance and then time a knife strike right before it gets within an arm's length of you.
If you doubt your skills at all, you can also hold down the knife button and just hope that you get it, which will work a good chunk of the time. Also, if you take too much damage from trying, you can reload after you get the trophy.