This is the thread I was talking about with the mage strategy discussion. Mind Blast and Sleep are priorities for defence, and after that, pick a couple of spell branches you like the look of and work on those. My favourite thing to do with mages is suck it up and get the useless frozen weapons so you can then get the freezing spell and Cone of Cold asap. Then just freeze everybody and get your rogue to do a crit-hit skill on them and they shatter. It is awesome. Even if they don't shatter, they're out of the fight for a few crucial seconds. Crushing Prison is also fun (and shatters frozen or petrified enemies), as is Petrify. Basically I enjoy immobilising people. After the Cold branch, I like a lot of the entropy skills and also fire. And Mana Clash is useful enough against spellcasters that it's worth getting its slightly crap prerequisites, but you can wait till the midgame to get it.

You do need some non-cold offensive spells if you're planning to fight revenants. Regular undead can be frozen (I think) but don't take cold damage, and revenants just shrug it off.

I set up a lot of tactics for my mages because mostly I was babysitting myself and Alistair, but that'll work a bit differently if you're actually playing your offensive mage. My favourite was to get Wynne to automatically do Fist of Stone or whatever it's called on frozen enemies (Wynne smash!) and also I set everyone except Morrigan to wait and not attack if an enemy was sleeping, to give her a chance to hit them with Horror, which does mega damage to slept enemies.

/tl;dr. Obviously I need to roll a mage myself.