1. A gnome warlock
2. A human warlock
3. A soulstoned human/NE priest or human paladin (pally can just bubble)
4. One other person. Doesn't matter what as long as it isn't a gnome or dwarf
First, zone into Naxx and you should see 4 triangular windows on the bottom that let you see into EPL. Have the GNOME warlock run to the very back of the window that faces the PLAGUE wing(wing with all the oozes, ghouls, and gargoyles)
Have the gnome warlock summon the human warlock, along with the other 2 people. A very important thing to know is that you cannot click the summoning portal after you accept the summon. Unless you have more than 4 people you should wait until everyone has gotten a summon before accepting it.
At this point everyone should accept the summon. Those that were summoned should be able to jump through the ceiling because half of their body has passed through it already. Jump forward and you should fall out of Naxx and down to EPL below. You will most likely die from fall damage, so this is where your soulstoned priest or bubbled pally rezzes you all.
Now have your human lock summon the gnome lock. NOTE: For this exploit it takes a very certain angle to work, i recommend summoning your eye of kilrogg and wait till you find a spot where it passes through the window half way before summoning.
Ok now for getting to sapphiron and kelthuzads lair, go to where light hopes chapel is in INSTANCED epl and you should find a snowy wall..i think, well ok to get over this snowy wall, sadly I Know i said no model changing but it's neeeded for this part, use dark portals to get up the hill. OR you can use Cheat Engine for Mountain Climber, but before you do I recommend you download the file in the Cheat Engine thread in Bots and Programs forum that gives the Cheat Engine window no name, ALSO, i recommend you use a process guard to protect cheat engine from being read by WoW.
Ok now after getting up go further and look for a sort of gap in the world, you should see some bones in it, thats sapphirons lair, kelthuzads is nearby.
http://youtube.com/watch?v=Bdl6T_pexIg
^ This video will help you find the gap for sapphiron and kelthuzads lair.
~Credit to nogg-aholic~