You're a Mean One...

As of 11.0.7, there is a new Express Transportation NPC called Father Winter's Helper that will take you on a sleigh ride to the Greench's location (you port directly once the sleigh leaves Ironforge or Orgrimmar).

Ironforge /way 31.7, 73.1
Orgrimmar /way 50.3, 79.5

The new event goes like this:

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• You fight [npc=54499] and he now has over 3 million health! (Note that he looks nothing like his former self "[npc=13602]".)
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• You do not need to be in the same group as the one that tagged him, simply fight the boss and when he dies you can loot sacks of the stolen goods in the cave (the one right behind him, stuffed with those Christmas trees, presents and what not.) The reindeer will automatically be credited to you, you can always talk to it too if it makes you feel better.
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• You do not need a tank, a plate DPS can take his hits but I wouldn't solo him because of his large health pool. Still if few in numbers be nice and throw some heal or two on the one taking damage.
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It's a daily, each day you do it you have a chance to loot [item=73797] from the rewarded [item=73792] -good luck and Marry Christmas!

He usually spawns at 30,60. Sometimes 30,55 then a bit south and closer to the quest giver (Snowman).
It's a random spawn between the 3 locations, but I've seen him spawning mostly at 30,60.

/tar The Abominable Greench
/run local n,t,c="The Abominable Greench","target","YELL"if UnitName(t)==n and(not UnitIsDead(t))then if GetNumPartyMembers()>0 then c="PARTY"end SendChatMessage("-> "..n.." is here! <-",c)Minimap:PingLocation(0,0)end

Make that advanced macro and spam it, once he is up you will make sure you and your
party knows he is up at your location! Just tag him with a low ranked spell and let him hit you until your
party members arrive at your location, as a 70 you can stand there for ages. ;)

He is camped by a lot of 70 so if you are lower level ask a pal for help.. try to group up with 5 others
and camp each spot, tag and tell party he is at your spot. If I am you don't help killing because it might
result you getting killed by a angry alliance party that only half got the loot and the other half didn't
because they were too far away. :P

(Updated, same deal only a bit better.)

If the Greench is already dead (by your group or another), all you need to do is talk to the reindeer, and loot the stolen treats.