Lead your shots. If you have a projectile, never shoot at her; even in close range you must shoot ahead of her. If you have a hitscan, wait until you have her in your crosshairs to shoot.
If the tip of his arrow shimmers, he is preparing an Impaler Arrow. Avoid being pinned to the wall at all costs; his Planted will do 2400 damage before you can even move again, and likely another 2400 to follow, killing anyone.