Juece:
Did you get the tail?
A method I found useful is to be lightly armored (so you can run fast) and have a high damage weapon (I used Black Knight Sword).
I dash in to destroy the crystal (stop short just right before to regain stamina) which will piss Seath off and get him to start screeching.
While he's screeching, I turn and sprint to the tip of his tail and start dicing.
Did you get the tail?
A method I found useful is to be lightly armored (so you can run fast) and have a high damage weapon (I used Black Knight Sword).
I dash in to destroy the crystal (stop short just right before to regain stamina) which will piss Seath off and get him to start screeching.
While he's screeching, I turn and sprint to the tip of his tail and start dicing.