I just bought this yesterday and love it, I've already done 3 of the origin stories and have gotten to level 7-12 on them. Going to try a Mage next, they look fun.

I've played through the game 3 times now, once with each class. I always used Morrigan, but never her shapeshift ability, because a mage with no spells and more health is just a really shitty tank.

I just used frost/fire/telekinetic weapons and the arcane damage moves, like fireball and Blizzard.

On that note, did anyone ever get the hang of the whole "spell combo" thing? Like casting grease and then fireball?

One is to cast force field on an enemy, and then drop a spirit cage on it. That creates "shockwave" which dishes hefty damage and knocks down all who stand around. The cage can also be used to shatter frozen opponents.

Another one is to cast vulnerability hex + drain health/mana, which basically creates "improved drain" and multiplies the effects by 2.

You can also cast Cone of Cold and use Stonefist (or deliver a critical with a non-mage character) to shatter the enemy. However, it won't work in every enemy you'll find.
You can check more of these combinations here.