Fire arrows aren't really the best for a single-player encounter with Moder. Their base damage is too low, even though she's vulnerable to fire. Try poison, or obsidian arrows from the Draugr Fang bow, which are poisoned by default. And put some work into preparing the battlefield. One of the vulnerabilities of the game is that if you spend enough time arranging the environment, things can get very easy. I don't mind that -- victory being the reward for taking the time to get things right -- but if you're insistent on hurrying and hooked on the rush of "brutal" combat, you may find it offputting.