i have successful done on ptr and yesterday on alive server with 5-melee group, my 5 paladins, in heroic.
my impressions.
this fight have 3 mini phases.
"mini" phases consist mainly in boss ability changes: you can continue attack him all the time.
-p1, typical "stance", same one you get when you engage him, consist in huge spell attack on tank+well of souls AE+ sort of drain life ability strongly related/chained with his AE ability(usually AE proc in place where is drained-target).
if you check with attention the ground, when he cast AE ability, a ground area (a perfetct circle) looks like pink colored: move out fast and place your grp in safe spot. my impression, this phase have a longer duration.
typically i wiped for drain-AE on prt, yesterday he gone down at 1st attempt, thx mainly normal latency on live server.
-p2 starts when he changes* face, he stops himself in a specific postion, dont cast anymore p1 spells and summon spirits. he dont cast the infamous drain+AE spells (yes the most dangerous thing in this fight for me), so in case you can heal hard (if you need to recover some health) or do high dps here .
-p3 he stops doing other abilities (p1 or p2), change to really different face(lol) from p1: its really important here stay out from face-front position. he cast a cone of luminous rays. in case you cant move out your party members get basically instant death( tank could survive, depends current health).

the main strategy for me is basically stay out from well of souls AE area and heal drained target and tank.
i taunt boss from drained-target too.
aoe in p2, and keep up dps on boss.
then find safe spot outside his frontal cone during p3, and dps.
remember mini phases rotation continue till he dies.
hope could help a bit