I had to get Morrigan's spirit-healing up before I could kill him, just so she could revive my ranged warrior and Alistair when they were killed -- which was often. I also had her on the other side of the wall out of the range of his freeze and other EVIL.
I think that with my rogue specializing in dual weapons this time around and just the experience of already completing the game, I'll be able to kill him earlier.
I did the "tank with potion" method: set your tank (Alistair in my case) with tactics "if health drops below 75% use a potion." With lots of potions, the survivability of the tank goes up a LOT. Once you burn through all the level 1 and 2 potions, change the tactic slightly to less than 50% otherwise you're just wasting potions.
and freeze locking has been somewhat nerfed since 1.02, but having Morrigan spam CoC is still a very useful way of dealing with Gaxkang.
There are a lot of other methods, of course, and it all depends upon build, but I had a strength/dex Dual Wield Rogue (which was far from optimum) but it's doable. I can't wait till try my current Cunning/Lethality Rogue...