This achievement is for destroying 500 buildings. This might come in time from you missing targets with launchers or attacks but that depends greatly on your accuracy. The game tasks you with walking from place to place often, so I took advantage of this time by strafing and shooting buildings with a rocket launcher on the way to the next waypoint. I got it very quickly this way and it doesn't require any extra effort since I'm going to the next story part anyway.
The achievement is cumulative and does not have to be earned in one stage, and it carries over between difficulties and other modes as well.