Use stealth to avoid being seen as much as possible, while using your precision arrow shots and magic arrow shots to take care of any enemies. After you get trapped in the space with the flaming arrows, run straight around the corner to a stone outcropping to use magic on, jump up and open the main gate, then jump down and use magic on the ballista, then run through the door to complete the level. Don't bother fighting enemies, it'll just get you killed.