I played a mage with maxed ice/earth magic with all the hex spells, the necro MP regen, and back up healer. I turned Morrigan into a hybrid support/healer but near the end of the game she was a full blown healer as my mage took over the support role and moved more into a damage role. Was a CC stud with Cold of Cone and Petrify and got the 250+ shatter milestone really early in the game.
I only had Sten, Dog, Oghren, and Zevran as extra characters and used Morrigan, Sten, and Zevran. Sten was a total beast in the tier7 set of armor I found for him midway through the game (forgot the name) and could hold his own solo tanking... not sure why so many people here hate him and use Alistar. Just get Sten to 40 STR as soon as you can and buff his Constitution and DEX and he's very solid. I couldn't stand Alistar so as soon as I got Sten I benched Alistar and stripped him of all armor and weapons and never used him again. His approval dropped below 10% after I let Redcliffe die and in result of other actions and it was hilarious to see him in all these "serious" cutscenes with no clothes on and his junk hanging out. I also made him fight naked and die in all the final battles so he wouldn't waste any potions. He got all pissy when I didn't pick him for the Archdemon fight, then I told him it was because I didn't like him and to "piss off" (direct quote). He just shook his head at me in disgust. It was hilarious.
I managed to beat everything on normal difficulty, including the archdemon on my first try. My mage was totally overpowered, soloing entire rooms and named monsters - sometimes it was just easier for me to solo them then have to deal with melee being cure sponges. A good strategy, albeit somewhat cheap, is if you're playing on PC is to use top down view and cast spells such as Blizzard through doors and below pillars so the enemy doesn't spot you or enemies appear on your radar so you know what you're dealing with. Makes things a ton easier, and even if an enemy or two snuck through, I still had Cold of Cone, Petrify, Misdirection Hex, etc to pretty much solo anything. Like others have mentioned, Mage is pretty broken in this game.
I read all 27 pages of this thread after I beat the game and fully agree about the cumbersome AI and tactics. I would constantly have to stop battle and re-direct my attackers to make sure they were attacking the appropriate targets and if they killed their targets they moved onto the one I wanted them to move onto. A queue system is desperately needed. It was also extremely frustrating when I would attempt to move my party to a specific location on the battlefield midbattle and someone would get attacked... they would stop and attack the enemy and i would have to pause the game and redirect them.