To complete this, you have to destroy every single building in the whole map, with those being touched by the map border not counting (and also taking no damage). While that might be somehow obvious, the other thing to note is that every bigger cylinder needs to be destroyed as well, with the majority being in the south of the map near your home base, others being more in the middle. Please also watch out for enemy counterattacks inbetween that are dangerous to your Katjuschas. Cover all 3 routes with at least one AT gun to manage that. To support the Katjuschas with leveling buildings, you can use the manual fire mode of AT guns as well as explosive charges of the engineers.