This is done in Mission 3 "No Man's Land". You will take control of Riley once at the beginning for a few moments where you can get 1 or 2 kills (L1 or L3 button) and then a more lengthy section later on where you have to sneak through the enemy camp. I can confirm that in the second section you can die and start from checkpoint and your kills will still count and I recommend doing this that way or you may wind up a couple kills short by the time you get to the end if you play too stealthy,