User blog comment:Arad1376/Moonlight Butterfly/@comment-3492791-20140708151821

Sounds like a bug to me (no pun intended). While it stays out of melee range for most of the fight, it will eventually land on the bridge so you can smack 'em around. It may have been trying to land and never got told to stop anywhere.

As to the fight itself, you should summon Witch Beatrice. Her summon sign is hidden under a bush beneath the stairs of the tower you climb up to the boss fight. She'll basically do all the work for you, all you need to do is block and dodge.