There will be a point in the level when you leave a building and enter an area where there is a dilapidated bus that you have to go through, once you reach the halfway point of the bus an RGP will hit the bus and a few enemies will approach. The building in question will be farther back on the left with concrete barriers next to it, once you leave the bus, the building will start to fall.
I got this achievement by lobbing a grenade out of the back door of the bus, just aim in between the farther barriers next too the building on the left, and you should nail them both. just make sure you don't leave the bus or the building will fall
Another method that worked for my brother was to look out of the drivers side door and you should be able to faintly see the flashlights of their guns off in the distance, just keep shooting at the lights until they don't show up any more. and just to make sure you can always lob a grenade over to them...
Hope this helps good luck!!!