Load up chapter 5 on easy and do the 2 little climbing sections. After that there should be a box in front of you with a Micro - 9mm on top of it, grab it. Kill all but one enemy with it in the next section and then purposely die so that you go back to the checkpoint where you had Micro - 9mm ammo. Repeat as necessary.
Also you should pause and save game after getting this so that the game keeps the medal points.