As you may know the requirements for The Immortal/ The Undying requires only to kill Kel'Thuzad.
The Immortal - Achievement - World of Warcraft
The Undying - Achievement - World of Warcraft
Step 1. Find 7-9 other people to help you. (or more for The Immortal)
Step 2. Have ever raid member download any hack that allows you to go through walls and go up. IE: Monstr Hack.
Step 3. Zone in to Naxxramas.
Step 4. Go down and you will be in an EPL looking area. Look around for a giant white mountain. Go to that mountain on your land mount, and go through the mountain wall until you know you can hit land. In the distance you will see a strange looking building. Go there
Step 5. Just walk into the building and you will be in Kel'Thuzad's chamber.
Step 6. Get fully raid buffed and act like your going to fight him for real, because you are.
Step 7. Don't die or this will all be for nothing
Here is the view of outside.
View of Sapphirons gate-thing with him still alive
Picture of event starting.
Picture of me running out and resetting him
I hope you enjoy this. This hack has a very small chance to be detected.