Hi Guys
I cleared this whole game up in one very fast playthrough but this achievement just would not pop for me even though I'd thrown every grenade I'd found and was picking up kills. After completing the game I tried all sorts of things such as choosing mission select and playing until checkpoint and repeating but it just wouldn't work.
I was about to begin a new game when I thought I'd give one thing a try which was the only piece of information I could find about this problem which some people experience.
Here's what to do: Load up the 'Last Stand' mission. You start with 2 grenades and 3 more on the desk. Get kills with these through the windows but be careful as they can slide way beyond where you want them to land. When you're out of grenades and hopefully a few kills towards the achievement, finish off the other enemies and you'll be directed to the roof to assist with sniping duties. There are two grenades up here that you can throw over to try and pick up some kills then start sniping until you are tasked with going down another floor.
On your way to your next defensive position you'll pass a room with RPGs in it. Collect all that you can and head to the railings. If you loose off your rockets at the side of the white tent you can kill several enemies. There are a few ammo boxes if you need to re-fill. If you gained several kills with the grenades at the beginning then you should quickly pick up your achievement, if not just keep firing that RPG as you have to defend this position twice which should be more than enough enemies.
I hope this helps people who had the same problem as me as I was starting to think it would never pop, Happy Hunting
