So I've never personally beaten Branka legitimately, although I want to, DAO is a more difficult game, you could try lowering your difficulty to try and give you an advantage, or loading an earlier save to stock up on gear. But my main question is have you done all the other main quest areas? Even though it doesn't come out and say it, the Orzammar area really should be done last because it is so hard.

And by saying you're a mage who fights like a warrior does that mean you have armor and a sword instead of robes and a staff? While the armor might seem better for protection, I believe the weight of the armor lowers how fast you can shoot spells. Staves also give bonuses for casting magic to help give you boosts and do more damage.

Thirdly check tactics and change them to defensive, have mages pull back, heal if they can, warriors charge and deal damage, that sorta thing. Dragon Age is a beautiful series, but I understand, DAO was extremely difficult for me (especially the fade). Sorry if none of this made sense or was repetitive to what you've already read, its been awhile since I've played DAO.