(I have searched, and found nothing of this method)
Characters:
Toon A - Level 80, who has also completed RAF recently.
Toon B - Level 1
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All you need for this is the X-53 Touring Rocket from RAF.
Somehow get the Toon B to Dalaran, whether it's through mageport, or any other way you can think of.
Of course, Toon A and Toon B need to be in the same party. So get that done, and meet in Krasus Landing. Toon A mounts up, and Toon B gets in the passenger seat. Now, my favorite place to do this is over in Sholazar Basin. So, go over there, and follow these instructions for whichever achievement you're looking to do:
[Going Down]:
Fly high up above River's Heart with the rocket, and look at your minimap to make sure you're in the center. Then, once you've reached the top of the sky, (lolwut) dismount, and Toon B will fall into the water, getting the achievement.
[Drunken Stupor]:
Right, now, make sure you brought some alcohol with you, preferably the strong beverages. For this achievement, Toon B goes down to the ground, drinks till he/she smashed, and gets back in the X-53 Rocket, then, you fly high above River's Heart, right into the center, fly as high as you can, then Toon A dismounts. Of course, you both will fall, either getting the achievement, or not getting it (if it's already been completed.)
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Note that this can be done in any body of water in Northrend or Outland.