The hardest of all the new celestial world bosses in the timeless isle is Yu'lon due to having to deal with the green patches of bad stuff, and the green wall that comes every now and then. I found out that if you pulled aggro on the boss while standing on top of the wall, but not enough to make him despawn, it'll negate all the mechanics of the fight making this a simple tank and spank. http://oi42.tinypic.com/20l00vn.jpg As you can see from the pic, I was tanking the boss just in front of the pet battling npcs.
To do this, just have a hunter or rogue MD the boss to you for the pull. Or just stand up there and taunt the boss to you when it is close enough.
This strat will make things much easier for people who need to kill all 4 bosses for the legendary quest and can't find enough people to do the boss.