If you want a full offensive build for a mage, I would recommend Blood Magic and Elemental. Blood Magic gives you some good options for keeping your HP up while still doing some damage and gives you a larger pool to draw power from, allowing for more frequently cast spells. Coupled properly with other characters, the Elemental build will let you cause some massive damage. Primal also works well for this. If you have enough points (which you will if you do every side quest you have available to you), I would go Force Mage for your third class, which will give you a few options for keeping enemies away from you and keeping them grouped together for you AoE attacks.
I've found that, in higher difficulties, Fenris is not strong enough to handle being a tank, so I would recommend using Aveline as your tank. I personally favor Varric as my rogue as he's more offensive. If you have the Exiled Prince, Sebastian can also work well if you want someone with a few more support abilities. Isabela can do some massive damage, but she can sometimes be difficult to keep alive since her health pool is rather low and she fights on the front lines a lot. It really depends on how apt you are at managing Aveline's tanking abilities. Lastly, since you'd be playing as an offensive mage, you'd need Anders to cover healing.
Obviously the characters you bring along with you can be affected by which quests you're doing and how much favor you want to gain with whomever. When I'm faced with this dilemma, I try to keep Anders with me at all times, switch Aveline for any other warrior, and switch my rogue out for whichever other character I need.
Hope this is helpful to you!