To complete this achievement, Elder Nadox should be kited to the very entrance of the instance with little to no dps placed on him. Any small swarmers that accompany can be aoe'd down in the process. After he has been kited to the entrance and threat has reached sufficient levels, DPS is initiated as hard as possible. The boss must die before even a single Guardian reaches within range for the immunity aura to activate, and with a good DPS group this achievement should be quite easy to accomplish.
NOTE: Since Patch 3.0.8 A soft enrage has been encountered on this boss. Once kited out of the room it will gain a 500% damage enrage buff.He can be tanked even in enraged phase though it really needs proper gear from tank and healer and the use of cooldowns).
Another tactic is to kite the boss through the door to his right, down the hallway overlooking the grounds before the second boss. Tank him in the farthest end of the hallway at the base of the stairs. Have a hunter cc the Guardians where they spawn or let someone offtank them and nuke the boss as fast as possible. Elder Nadox will not enrage if he is at the base of these stairs.
This second strategy is much easier than the first, especially as there is no enrage on the boss. We just completed this achievement with a group of 4: a boomkin was tanking Elder Nadox at the bottom of the stairs with the heal and a second dps and the tank stayed in the room to aggro the guardians. Took only two attempts with this strat and, I must say, this was a lot of fun!
There is another strategy that is much like the 2nd one, but involves different placement. In the room to the right of the boss if you're looking at him, there is a path that has a ramp going up, a middle platform and a ramp leading down to another platform. What my group did, was tanked the boss on the 2nd platform, healer was on the middle platform, and the guardians were tanked on the entry way to the path. The distance between where the boss was tanked and the guardians were tanked was more than enough to keep the shield off him. We had a druid tank the guardians with the healer mainly focusing on him, a DK tank the boss which seemed to work, he was blood specced so he could gain his own health back. Hunter MD'd the boss to the DK so there wasn't any kiting involved. We just burned him down and reaped the rewards. :)
Alternative Strategy Edit
Tank the boss right where he stands. Have one DD wait for the Guardian to spawn and kite the Guardian out of the room. As soon as the distance between Elder Nadox and the guardian is 30yd again, he'll be vulnerable to attacks again. The Guardian doesn't hit very hard and is vulnerable to slow/stun effects, giving your group plenty of time to kill the boss. Easily did this with my rogue; any other class could do this too.