Help Dragon Age 2 choosing between Mages and Templars!!?

Okay so I am playing DA2 and I play a rogue and I am at the point where I have to choose to side with the mages or the templars and I chose the mages and I am running to the gallows and I have reached a point where i have to fight like 10 of those reavant things and a big demon all at once, i only have 7 potions and all my guys keep dying i cant seem to get past this part OH! and im playing it on casual difficulty, any suggestions would help i hope