I made a pug for this some time ago, we were around 10-15 players. What you do is have EVERYONE download dbm/bigwigs for naxx and type /range 15. We tried /range 10 but it didn't work so take 15 for safety. Tell melee to not even jump the platform and go wank in a corner. Note that they still need to spread. Have your ranged spread out before thaddius engages. If people don't have downs they should find a safe spot in time. Then tell them to not move an inch and it's all good game.