Fight him pretty much where he is so that you are equal distance from either side of the room. The named has a buff on himself that gives him a % chance to spawn adds every time he takes damage with a 20ish second cool down. There will be a red message when it happens. It has very little HP, so burn it down fast. If you do not kill it, it will grow into a Velium Warrior and start casting AE's that could end up being a fail condition for your raid. (This is not a full fail condition. If you burn down the Velium Warrior fast enough, you can recover.)
At specific % of health, the named will spawn a small copy of itself on either side of the room calls a "Smashing Machine". You have to drag the named over to it and he will destroy it. DO NOT get too close to it or it will kill you. Everyone else hang back and let the tank drag him. Once its gone, continue to burn and watch for the next one which will most likely be on the opposite side of the room. Rinse, repeat, dead bad guy.
**Challange mode is stopped with a dispel or absorb magic. Serene Symbol works too.
Challenge Mode
Challenge Mode is pretty much the same encounter as Normal Mode with a higher % chance to spawn adds, the addition of a group of Crystalline Adds that spawn every 45 seconds or so, and harder hitting aoe's and melee damage from the Ancient Sentinel. In the new group of adds is one specific mob that has "prelate" in its name. Make sure this one dies first as it will start healing its allies if it is up for too long.
If your raid is flawless in Normal mode you can pretty much clear Challenge Mode if you have enough AOE DPS and you can kill the shardlings that fall off of the named before they grow larger.
Good Luck.
Ugh... I really hope that was not intentional. I do not remember him healing on the curse before.
on a pull after gu 66, challenge sentinel was healing 2% of its max hp per curse. yeouch
Some additional data from Soraya.net:
Class: Warrior
Max Health: 952,592,618
Max Power: 12,581,800
The loot table is also listed, with percentages for how often a given loot table is used, and the rates for drops from each table.
We got him last night 
The key, for us, is the curse, ensure that the curse is handled and then you will be fine. Make sure the tank communicates when he will stop for cures before moving off.
We have our own strat which seems to work out for us which got him, not the fastest (19mins) but its done!
The smashing machines I believe are also stunning him in place so he cant move, curses are generally being taken care of quickly.