First make sure you line up your raid at the top of the elevator shaft, nobody should cross the red line demonstrated on the screenshot above.
Priests should be prepared to life-grip anyone who falls, while everyone else should be ready with their "/point /laugh" macros.
When the boss arrives, everyone should quickly move onto the boss and carefully position themselves away from the edges. Use parachutes or other slowfall abilities with care, if you screw it up you'll either splat anyway, fall into lava or worst of all aggro Nefarian and Onyxia which is only funny to those still at the top of the elevator shaft who can run to safety.
Tanks should make sure that they keep aggro by staying on the boss until it reaches the bottom (or if you're feeling like a crazy risk taker you can jump off a few feet early).
Healers should make sure to keep the raid topped up by staying on the boss until it reaches the bottom (or if you're feeling like a crazy risk taker you can jump off a few feet early).
DPS should make sure to maximise their damage output by staying on the boss until it reaches the bottom (or if you're feeling like a crazy risk taker you can jump off a few feet early).
Hunters can feign death if you want to do something pointless and silly. It's expected of you after all.
Finally once the boss reaches the bottom the raid should quickly walk off of the boss and onto solid ground. Make sure to res and mock the dead before moving on to the trash secure in the knowledge that the most difficult encounter in the raid dungeon is now behind you.