This is coming from someone who has slogged through the entire game and all DLCs on Nightmare as a Mage. Having your main character as a mage is beyond retarded. If, for whatever reason, you don't want to be a Rogue a 2H (Everything in Strength) or DW (36 Dex, rest in Strength) Warrior is a decent option.

You should have no difficulty breezing through the game on Hard or Nightmare (Your party will probably die left and right on Nightmare, but you won't). If you do manage to get hit by something like that (or are being chain Overwhelmed by Drakes/Werewolves/Spiders), simply have Wynne or Morrgan Force Shield you, and you're good to go. Spells are still dangerous, of course but with combat stealth even mages are completely trivial unless there's more than one Elite mage and the other one slaps you with Crushing Prison while you're killing the first. This makes you literally unhittable by melee attacks and bows unless you're stunned/knocked down. Max out the stealth line as soon as possible, as that lets you stealth in combat on a hilarious 10 second cooldown. Pump every single point after you've reached those goals into Dexterity. Your goals are 20 Strength (Tier 7 Light Armor) and 22 Cunning (Max Traps/Coercion).

Best warrior build dragon age origins