I am not much of an achievement monger... there's a few I'd like to complete that involve interesting titles or a decent mount but on the other hand I am in no rush to complete them or do research into how to complete them.
In fact usually I discover an achievement purely by accidentally fullfilling the requirements.
The same thing counts for the achievement 'going down' which I now finished on all my characters after accidentally discovering it on my hunter.
For mages and paladins there's of course the age old method of hand of protection or ice block which works 100% of the time but for the other classes things looked a little more difficult.
Turns out... not so difficult after all:
What you want to do is get a port to Shattrath and go up the elevator on the scryers tier (not aldor's tier... it's too high).
Once you're up there look down from the right side of the elevator platform (the platform not the elevator itself) and you will see a bunch of archery targets (3 or 4 if I remember correctly).
Simply mount up at this point and run towards the archery targets below... you will drop down scryers tier and if you land close to the archery targets it'll eat up about 95% of your HP but it'll also complete your 'going down' achievement.
So really there's not much more to it than to drop off scryers tier on the right hand side towards the archery targets (don't jump or you'll go splat). Note that I've only tried it mounted so I could cover the distance to the archery targets but I would assume it'd work without mount too.
Maybe this is already a well-known fact but since I accidentally found out and some paladins are still selling (yes, selling, sad isn't it?) hands of protection on my server for the achievement I figured it'd be worth the post.