Hagravens can be a real pain. Remember that you can dodge the fireballs by moving erratically, and you can dive behind rocks or trees to avoid the blast as well. As a ranged character, you are likely to be able to find a spot behind cover where you can fire at the hagraven, but the hagraven can't hit you back.
If you insist on beating it "legitimately", your best bet is to gain a few levels and get some fire resist gear or potions. Use some poisons on your arrows, and apply restore health potions liberally. Remember, you can instantly drink as many potions as you want, so you're only limited by the number you can reasonably afford/create.
Another good strategy is to hire a melee companion, who will gladly run in and provide a distraction while you add your damage. Just remember that you can't get a sneak attack on a creature who is in combat with your companion, so try to get your sneak attack in first, before your companion runs in as a distraction.
Essentially, use everything you have available to help you and be creative. Each individual aspect of Skyrim's combat may be one-dimensional, but there is still plenty of strategy because there are so many options.