equip your best weapon, a slow weapon is good with a jumping attack to deal a large amount in one hit, and a light weapon is good to continuously deal damage as long as the boss is targetable. under 25% equip burden is recommended.
wait until gwendolyn starts firing arrows, then run up and attack.
you can use a greatbow without any response, upon entering the fog gate move forward to at closest 6 pillars away from gwendolyn and fire away. the hawk ring will help deal more damage. for other bows, poison arrows may be necessary. it will take 7-8 shots to poison gwendolyn and will require several rounds. bring lots of arrows.
for any magic, you will need to get closer. you can either get close enough to trigger a teleport, giving you a few free attacks, or you can sit far back enough and dodge attacks. homing soulmass is great for getting some extra damage in as you can cast it before you run up and cannot be dodged.
if fast enough, you can use dark bead to stagger her, allowing you to cast up to three spells before the boss teleports.