You can easily reach Sapphiron & Kel'Thuzad without having to kill a single mob in Naxxramas.
I won't give you a step by step tutorial, since this is fairly easy to figure out. This only requires getting threw the instance floor (model changing trick), then once in instanced NPL, find a way up to the snowy mountain east of Naxxramas (hint: use the Campfire > Darkportal MC)
I'm not sharing my edited naxxramas files, you'll have to do the job yourself, here is what I 'changed' to allow this trick:
World\wmo\Dungeon\LD_Stratholme\Stratholme_raid_071.wmo
World\wmo\Dungeon\LD_Stratholme\Stratholme_raid_072.wmo
Replace / remove those to get threw the entrance's ground.
Here's a video I made last month to show to my guildmates.
/Enjoy