All right guys. There is a hidden Feat of Strength achievement in Tanaan Jungle, called "Predator" ( Xemirkol - Wowhead Search )
Many people believed he would just randomly spawn, or be an extra rare mob. I've been sitting for a week at the reported spawn location, but he did not spawn, no matter how long I sat there. And after finding out how to MAKE HIM SPAWN, I can only facepalm for my wasted time camping the place. To prevent you from making the same mistake, here is my "How to" guide (short version).
He will not just randomly spawn, no matter how long you sit there. neither will you randomly get ported to him using the crystal!
1) Kill every Rare mob in Tanaan, and get this achievement:
Once you have completed it, you should buy the Exalted crystal that teleports you to a random rare.Revered will not work.
Now go close to his spawn location (all the way north, where we used to enter Tanaan at 90) and use your crystal. The crystal will teleport you right in front of the Rare mob "Xemirkol". He has over 3 million HP, and hits quite hard. You will see that the edges of your screen will be misty-purple. This is some sort of scenario. He will NEVER be up on that roof outside the "scenario", so DON'T BOTHER CAMPING.
Except melee attacks, the boss has 2 important abilities. One will create a purple shadow pool, stunning you for about six (6) seconds. Avoid at all cost, because he hits really hard. His second ability is something like a green chaos wave, which will attempt to throw you off the building.. If you get thrown off, the fight is over.
What if I get thrown off or die? Simply resurrect, and try again. It seems that you can have 2 tries PER realm, after that he won't spawn for a while anymore.
Your crystal will have a 15 mins cooldown. You can simply bypass this by joining a quick 2vs2 skirmish and leaving before the game even started. You will get a 1 minute deserter debuff, but the fight will take longer than a minute.
If the crystal starts to "fail" to find prey (aka, you tried to much), simply hop to another realm, and repeat. The boss "spawn" is tracked on your realm, likely with a cooldowm to keep it hard. So this makes it a semi-exploit.
Once you've killed him, you'll get this achievement to show off:
This also shows the misty purple shadow at the edges of the screen. It's the only phase in which he can spawn. Really never bother to ever camp him, he will NOT spawn if you have't killed all other Tanaan rares.
I hope this cleared up some doubts, and also speeds up the process by the skirmishes / cross realm trick.
I would suggest you to bring max dps for this, or at least properly buff. Not sure for plate, but on cloth he hits really hard and you kinda have to kite.
When he is above to cast the chaos wave, make sure you are against a wall, or behind his back, it's a one-shit mechanic that will end the fight immediately, even at 1% boss health remaining.
Congrats. If you can't manage to do it, I can also boost it for you. Just add me to Skype.(I don't mean to advertise, a mod can delete this line, in inappropriate).