I did it with the greatsword +10 and yellow pine resin and killed him/her/it the first try. The key is to just keep moving and time your attacks. I used all 10 of my flasks (Havel made me use 2...bastard) but just took my time and I finally beat it. Once they split up, wait for them to get close together so you can attack both at once. Patience is the name of that fight for me. Some say bum-rush the sh*t out of her but I say patience and strategic rolling.