Gaxkang has a very large damage output on higher difficulties, and it is recommended to ensure your party has staying power for this fight. Having two party members with the ability to dispel magic (Dispel Magic and Cleanse Area both work) is very helpful. Curse of Mortality is very helpful, as it prevents him from healing with Drain. Magebane poison can help reduce his damage output significantly.One easy way to kill Gaxkang is with the spell Mana Clash. If successful (he can resist it; consider using Spell Might to make it stronger) Gaxkang will immediately take over 300 damage and change into melee form, which make him far less dangerous compared to his arcane horror form. Once he has regained enough mana he will change back to Arcane Horror form, at this point one more Mana Clash should be enough to kill him. Another good way to deplete his HP is to use Glyph of Neutralization which dispels all magic in the glyph's vicinity. While the glyph is active, you can wail on Gaxkang all you want without having to worry about status effects. If you have a warrior based party, Magebane on a dual wield warrior is highly recommended. As he is vulnerable to fire, another possible strategy is to pick 3 melee and one mage keeping Flaming Weapons up (be careful to keep some distance between the mage and Gaxkang as he will try to dispel it by mass dispel). Together with common fire enchants and some resistance vs spell and cold it should become an easy kill. Cone of Cold, while dealing little damage against Gaxkang's high frost resistance, has a good chance to freeze Gaxkang for a second or two, reducing his damage output by that much more. See: Dragon Age: Origins