here is how i did it, took me 5 total tries.
first 3 tries were all about learning the fight's mechanics. i felt a little defeated and chagrined to admit that i CONSIDERED dropping it from nightmare. i came back tuesday after work and i used the only respec potion i had on me (thank goodness i had it). i turned my rogue into an archer.
4th try i had a bs death to my healer, 5th attempt it was gravy. long, slow gravy
my party:
aveline - tank
varric - ranged rogue
neverdawn (my rogue) - ranged rogue
anders - healer mage
i set my tactics to be nothing more than "atk enemy with lowest health" for all but aveline, who was "atk nearest target"
i summon the dog everytime as soon as an add phase ends, mostly as a soaker for some of the boss's abilities.
i pause constantly. CONSTANTLY. all of my team is on HOLD POSITION command almost at all times. (and 100% throughout this fight). i use anders STRICTLY FOR HEALING and autoattacking.
boss's mechanics, as i interpret them, and how i dealt with them:
phase 1: boss warps to set locations. as soon as he's attackable/hittable, taunt with aveline for initial (and permanent) aggro. keep aveline right up in his grill, use bolster every second of the day and stonewall everytime he's about to atk. her or do a point-blank-area-effect ability. repeat this with her THROUGHOUT THE FIGHT besides during add phases. always move the 3 non tanks AWAY from the boss, preferably spread out.
phase 2: first add phase. first, manually move everyone behind pillars, in this phase, i put one person behind each pillar. after the ae, the adds spawn. the adds are very spread out. use the mage and the two archers to take them out ASAP. use rain of arrows right off the bat. if any add stuns one of the ranged w/their ability, have the other two kill that one IMMEDIATELY. have aveline bolstering the entire time to have full energy when boss is live again, but otherwise, atk nearest add. after the adds are dead, use mark of death on the boss & every possible dps ability w/the two rogues. anders just autoattack, always save his mana for heals.
phase 3: when boss goes live, he has a couple new tricks. he will pick a random group member and turn into a ball and try to roll to them, damaging/knocking stuff down in his path. you need to identify immediately who he's rolling into. MOVE THAT PERSON to a safe spot, even if it means next to one of the other two ranged. but i find a new spot for ranged if they're ever bunched, so as to not have to move them as often (if there are two in same spot, you have to move both). aveline constantly chases after the boss, bolsters & bulwark's (stone walls) the entire time. she will halt his rolling, too. also, his rolling can run him into pillars, which is nice, cause you don't have to move your ranged.
phase 4: 2nd add phase. this time, they're not as spread out, but instead, in groups of 3 on the north and south side and a couple sparsely occupying east/west. for this ae barrage, i bring EVERYONE to the same pillar, so that the mobs are more controlled being in packs. after the ae, i send aveline to taunt the 3 pack nearest the group (so if you went to the northeast pillar, she went to the north 3 pack). the ranged quickly kill the individual ones near the group and then they AE the 3 pack in the south, kill them FAST. then, kill the individual ones on the far side (west in our example), and then burn down the 3 that aveline's been holding. mark of death on boss, lay into him, use your stamina pots for this phase if needed to get as much damage as possible. then spread out again.
phase 5: the boss now has a suck-in ability that is LOS'able. everytime he balls up to do this, get your 3 ranged out of LOS. stop atking him. when he's done, make sure to stonewall aveline because he'll AE the immediate area. as soon as the sucking in is done, you can also immediately jump out from behind the pillar and start drilling him. the ae is a small range.
burn his ass down
please let me know if i mixed up any of the phases...
anyway, be smart w/your heals (mana potion quanitity dependant), use health pots on aveline if needed, time your barriers from anders on aveline smartly if she wont' be able to stonewall before a big hit, etc. NEVER EVER LET YOUR RANGED TRIO GET HIT, especially anders, by anything.
long fight, but hella satisfying when done.
other specifics for me: i had 10 restoration potions, 6 healing potions, 1 elfroot, 5 stamina, 6 mana (and 2 injury kits but i never need them... but if you lose a rogue or something and have to rez them w/anders, use the injury kit on them otherwise a mistake could get you 1-shot)
don't forget to use that dog! he can soak a lot of damage.