As a member of gamers with OCD... for the LOVE OF GOD please round the potency skill Cap of rogues up or down to increments of 5. It's driving me insane.
to kill the ghost samurai just use 'stun' everytime he uses the buff to do extra damage. you'll do about 13000~15000 damage and he wouldnt be able to use his buffed up attack on you. repeat that process and he'd die like nothing.
Use your special moves wisely and you should make him no problems!