What strategy should I use to beat a Hagraven as a Sneaky Archer?

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.

When I played this quest on my first low level character, I climbed up on the rocks, just right of the log bridge and fired arrow after arrow as she wandered around her little plateau searching for me. Took quite some time and a lot of arrows, but the rocks made good cover from her fireballs

Since you're a wood elf and archer, you should use Command Animal to provide a distraction. YIf you want a stealthy, ranged follower use either Jenassa (Whiterun) or Faendal (Riverwood). If you want a follower who will distract and absorb damage, use Vorstag (Markarth) or Argis (Markarth). It's not at all true that you can't score sneak bonuses when your follower is engaged in combat. Drop into a crouch immediately when you go to fight and stay crouched until you've been seen and need to run away. I recommend waiting until night (sneaking bonus if it's stormy out) and using whatever cover you can so that you can make use of the sneak critical. You only do sneak damage when your person is hidden, or your enemy hasn't found you yet. Sometimes it glitches a little.

You can sneak up with a sneaky follower and fire off a few arrows whenever your indicator says you're hidden until the hagraven dies.

Or: You can sneak up with a beserker type of follower, who will take attention away from finding you and cause damage while you wait to become completely hidden again. Don't get into melee combat, if you are found, run away and wait until you're hidden and can sneak back.