first got to quick play instant action select the lv hoth then select the options menu and 500% then go up to unit count and select a small number 1, 2 , 3 , or 4then play the game as the rebels select the vanguard and pop up at a command post with the taun tauns/snow speedersget in or on one of them then fly or run over to the At-Ats since the unit count is a small number u will only see a few people allowing you to go to the At-At and fire your launcher at its neck 4 times and then kill yourself and do it over again

Even though there is only a small number of people they will still take all of your command post so u might have to go take them back and if you get bored and want to finish early then just take their command post and the game will be over allowing u to keep your Demolition points
Hope i helped