RESIDENT EVIL RESISTANCE

Get the RPG from a Supply zombie or buy a grenade maybe 2 if possible from the crate. Stick with the squad a play as normal. The moment you get to an room full of zombies throw or fire weapon in there. Another way to get this is try and hold the weapon till the end of the level when you open the door. The Mastermind can spawn multiple zombies in very quick and wait for them to group up and use it then. The last method was the way how I got the achievement. It's best to wait for zombies like dogs, crawling or normal zombies, they are the weakest and die from a grenade no problem. The achievement pops immediately.

Easiest way I done this was with an RPG that I got from a supply zombie. Make sure to only use it when you see alot of enemies together. You can also have a Jan give you an RPG from her skills of special weapons from the box.