I actually was terribly aggravated by Vendrick until a friend told me a somewhat cheap but rather effective tactic he used. If you stay behind his left leg at all times, there is no way he can hit you with any of his attacks. All you have to do is just wait for him to attack, and then you attack while he's recovering from it, and repeat. For me though, hardest boss probably was the Smelter Demon (at least until I chickened out and used a Gyrm shield)