Vientos!

Tip of the year:

This achievement is actually piss easy once I figured out how to trade effiecently :)

Gather a friend or two, and enter a new raid instance, like Siege of Orgrimmar. When you die and release, you will spawn immediately and without a corpse run. That means: No ress timer, no ghost run.

Stand at the point of ressurection and let your friend kill you. If you are more than 2 people, you can get double/triple etc killing blows instead. Just spam the Release button and you will spawn at the very same point again, making it easy for the person who's gathering, since you can start "loading" a jump or sweep directly after they die.

With 3 persons in the party, this will be done in a couple of minutes without a problem. It's better to let one person complete the achievement first, so you don't havce to wait for the item's cooldown to finish.

Was furious when I saw this achievement, but with this trick, I just wished there was more in the series :)

Good luck! :)

Dueling seems to be the only option to make this work now (2015). Being in raid or just any PvE area makes it impossible to "kill" the other person and get credit.
However, you don't kill your opponent anymore (becasue you are in duel stance), so you can do the achievement everywhere you want.

But of course there is another downside, there is now a shared cooldown of 1 minute by all the costumes, and your opponent will loose the costume. So after every "kill" you have to wait until cd is ready and your opponent can wear the costume again.

Best solution is to get a group together, kill people one by one, so by the time you done the others, cooldown on first should be ready again.

edit 1: Apparently holy priests do die on each defeat, so make sure not to do this in holy spec
edit 2: Because of these changes I am wondering if the achievement can be done now when Day of the Dead is not up, will test this out in a few days

Completed by killing 20 other persons where both is using one of following:
[item=116856], [item=116889], [item=116891], [item=116890], [item=116888]

Using the item grants you an extra button with abilities that you can use on another player using one of the 5 items. The ability your button will cast is randomized with each use:
[spell=172057] - the weakest ability dealing 10% max hp.
[spell=172602] - the medium ability dealing 20% max hp in a 4-yard cone.
[spell=172603] - the strongest ability dealing 30% max hp, jumps 7 yards forward, and dealing damage on impact.

The items can be aquired from [npc=34382] for 100g.
Check the different maps on his page, to find his exact location (he can be found in more than one graveyard).

Note 1: Places which are sanctuary like Dalaran, making it impossible to damage people. And therefore you cannot complete the requirement there.
Note 2: To avoid ress time, you can do it inside an instance. I did it in Siege of Ogrimar. And we got a kill every 5-10 seconds. When I died I lost the extra ability, so we had 1 person completing the achievement followed by the other.