I beat this fight first try on hard.
Here was my party:
Hawke - AOE specced (Force tree, elemental) , heal
Aveline - Tank specced ,
Anders - Pancea specced healing mage , barrier
Varric - Ranged DPS
Picking the right party composition is key for this fight. If you can, having 3 ranged characters in your party will help a lot by avoiding cleave damage and the vacuum attack.
Also having a properly specced Tank is vital. I noticed a lot of people saying they are having trouble, but their party's tank is a 2h warrior. Having Aveline dedicated to Tanking talents made really easy to keep up heals. Use stonewall & Barrier (mage cast) whenever the cooldown is up. These abilities allow time for heal / potion cooldowns to run out, while the tank is safe from damage.
Important tanking talents to have: - Lvl 2 Stonewall - Putting 2 points into this gives your Tank an inuvlnerability mode on a 10s cooldown timer.
- Immoveable (Aveline Only) - Really helps her stay alive
- Turn the Blade - sustained tanking bonuses
- Shield Defense - sustained tankin bonuses
- All the passive tanking talents
Another key is having multiple heals availble. Between potions, two single heals from my mages, and AOE heal from Anders I had 4 potential heals on my tank. If my ranged took damage I could always run them to Anders to gain HP regen. Use your heals smartly! Don't waste a heal on your ranged dps when they can use potions. You will likely need all your heals for the tank.
Tactics:
VacuumKeep your ranged at range to avoid the vacuum. I actually didn't know about the pillars when I beat this, but that would have made things much easier. But using stonewall during the vacuum helps keep your tank up until your healers can get back into position.
Ball AttackWhen he does the ball thingy, see what direction he is going to roll and move your characters out of the way. I never had trouble with Avaline taking too much damage from the roll, so I just kept her on the boss, but you can have her dodge too.
VanishWhen he vanishes, get ready re-position all your characters and send your tank back on the boss. This is a good time for your to use taunt. If you're having aggro problems when he uses this ability, have your ranged members restrain from attacking until your Tank gets in a few hits.
MinionsBurn these guys down as fast as you can while using effective crowd control. You have a limited amount of time before the boss comes back up, but you should be able to kill most of them before he does. They don't take much to kill them, but they are all ranged (correct me if I'm wrong).
He summon minions at 75%, 50%, & 25% for me, but I would need others to confirm as well. The boss used a vacuum and becomes unable to be attacked right before the minions come. Try to move your ranged characters away from the center, and keep your tank on the boss. The minions spawn all over the room, so its likely that some will be right on top of your guys. Have your ranged characters stick together and systematically take the adds out. Be smart with AOE spells and crowd control. Your tank should suck up about half the guys if you get your taunt right and have the rest of your part on the edge of the room. Kill the minions on the tank last.
Its a really tough fight, I've played a bunch of MMOs so I felt right at home. But you'll have to pause the game alot to heal at the precise moment. I hope this helps some of you who are struggling atm. Unfortunately I don't think you can respecc or change your party in the deeproads so if you are having a lot of trouble with this fight, it may be wise to load an old save and start the deeproads over again.
I'll try to record and upload a video in the next few days.