First off, if Gaxkang seems difficult, well, he's supposed to be! This is /highly/ optional content, and thus a good place to sneak in an extremely tough fight; after I beat Gaxkang, my character record changed "Most Powerful Enemy Killed" from High Dragon to Gaxkang. Don't let his non-enormous size trick you.
Not that it sounds like you're under the impression that Gaxkang isn't difficult, mind.
Looks like you're level 15 now; frankly, you might just need to wait a bit before you're ready to challenge him. I beat him, barely, at level 17, with a high-DPS melee rogue, Sten, Wynne, and Leliana (at normal difficulty on PC). Leliana and the PC both had the ranger specialization, so they each summoned a wolf to assist. (Bears tend to get caught up in small spaces.) The PC and Sten stabbed at Gaxkang from opposite sides (so that the PC was getting backstab bonuses most of the time) while the wolves nipped at Gaxkang as well. Wynne just healed Sten as often as Heal could recharge, and cast Rejuvenation (health regen in combat) whenever Heal was cooling down. Every few cycles of Heal, she would manage a Group Heal to deal with any AOE damage that Gaxkang put out, particularly from his Cone of Colds. And I'm sure I went through a bunch of health poultices, too.
Oh, and Leliana stood in the corner with her bow, being mostly ineffective. She was basically there for the wolf, and also just because she happened to be in the party when I stumbled across the door.
The fight probably took 45 minutes in the real world, including pause time -- very five steps forward, four steps back, because of Gaxkang's healing. Wynne went through at least ten lesser lyrium potions -- I had a lot of lyrium dust, so she just kept crafting them as needed. I think Sten was only killed once, but I was able to use Revival to get him back up.
The biggest danger, I found, was Wynne getting caught in the Cone of Cold blast due to the smallness of the space. Make sure that wherever your tank is standing, Wynne's not in a line with him and Gaxkang.
Good luck!