Velvet Assassin

You first find people who you can trigger the grenades on during Lv 3. To do that, you have to find some one who has a grenade hanging from their hip (looks like a key chain) and sneak up to them. Then you push B and get to a safe distance because it can kill you too if you are close.

Level 4....there are 2 chances and 3 in level 5...all done.
WoooHoooooo.....
(nothing in level 3 that i could find)
You can reload level 4 from the start menu and just play this and the next level and you got it !

I have found on many levels some people have a Frag on their side level 4 as Major said had 3 for me and 5 had 3 for me as well as 7 if you see a Guy with a frag then all you need to do is sneak up like normal and then instead of A to kill you press B and run away. Also, I want to confirm that you can not die and Reload checkpoint i have gave this a shot no luck. So over the course of the game you need to do this five times. You could always save it for last and load level 4 and start from there!