My urinalysis on the situation is that, well ... :D

K.

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.

Use poison arrows vs moder , or obsidian , have reduced damage with others ( frost and silver ) , fire arrows do very little damage in that part of the game so nope

thanks for replys i noted all you said thanks you my friends.

Scrub my former advice to use frost arrows....that's really stupid advice, haven't had my morning coffee yet. They aren't much good against Moder. Needle arrows would of course be best but you probably won't have them yet. Obsidian, poison, or needle, preferably from the Draugr Fang since its output is always poisoned.

Btw for anyone who was wondering, the poison from poison arrows stacks with the poison from the Draugr Fang, making them technically the most powerful arrows in the game shot for shot. Needles still out-DPS them though, especially if you have high bow skill.