Velvet Assassin

You find the flare gun pretty early in the game (around level 3-5) and you get a lot of ammo through the game. You don't have to use all in a rush because usually items carry over but if you use it every time you get any ammo you should have enough kills before Lv 6/7. Every 1 flare gun shot is a kill as long as you hit them.

This achievement is best done during mission 5 (Leave A Light On - Docks). If you don't get all 10 kills before the end, you have the next mission to mop it up.

Here's a walkthrough with the collectibles bolded. It isn't as good as the old one, but it's still decent.

http://wikicheats.gametrailers.com/Velvet_Assassin_-_PC_XB36...