In my finished game, I had mages at my disposal (as opposed to the Templars you get for wiping out the Mages Tower) but I only really used them when I was fighting the Archdemon.
Also, I was playing on Normal the whole way through and used mass area spells like Sleep and Horror in conjunction with each other; made things simpler in that unless there was an archer or two hiding in the background, all I had to do was use Cone of Cold to stop whoever was uneffected by the Sleep/Horror combo and if that was enough, I just stepped in and started wailing on the rest of the enemies.
I'm assuming you took out the Darkspawn Generals first before you fought the Archdemon because I'm thinking that that could also affect how hard it is to fight the dragon at the end...