I am so very, very tired of saying it and so this amount of frustration is very much not directed just at you but...
The undead dragon should be tanked on the large center platform!!!
There is no reason not to and every reason to do so. Gear has invalidated the damage of the skeleton explosions to the point that all you need is maybe one healer casting a single heal and they can all just pop, it doesn't matter.
Tanking it on the center platform (I usually stand on the square to the north of the center and keep the dragon there) means less time running, no interrupted casts for casters/healers who started casting as the boss became targetable and full positional accessibility for melee no matter the level of the purple floor poison.
I do this whenever I tank that place and I've never seen a wipe. It isn't difficult, it doesn't "strain the healers" as some people try to claim. It's just convenience.
For the love of the twelve keep it centered.










