kk, you make an explosion (For instance), and when it explodes you right click on it in the event editor, animation-paste image into background-not an obstacle. This will leave the explosion there but you can walk over it. Only works with background objects.
Random terrain? You have a tree. set x position random (800), y position random (600) or something. Not that clever, but it makes it random.
Make two objects, your destruction range (Crater in the ground) and your shot.
When the shot hits the BG, create crater object at 0,0 from the shot, paste the crater into the BG (not an obstacle) then destroy it. I believe this works with invible objects, but I cant quite remeber, but if it doesn't, then you'll need to make your crater object is the same color as your non collidable BG.
For random terrain, draw a bunch of hills or such as active objects, then just make a group of event that randomly changes their X,Y coords then paste it into the BG.