well, i think that the best weapon could the lance, u should attack everytime that a chance appears, and block or evade every 2 or 3 attacks.
a nice way is fishing it at the beggining of the quest cause is easier on land than underwater, theres no much to say, his attacks are very simple, just block or evade every 2 or 3 attacks, thats all u should be able kill him like that