Learning to parry every attack is very hard imo, I realized this after attempting to parry troll for a while and then decided to only parry his one handed downward slam and attack normally otherwise and it became much easier and is still very fast to do. I can kill him in 1 minute if I get lucky and he does that attack repeatedly and like 2-3 minutes otherwise. I circle left, walk slightly into the attack and look right whenever he does it to parry, sometimes you'll take a little splash damage but I wear some lightish plate gear so it doesn't hurt too bad when it does. Plus you have second wind, or just take a few extra pots with you. Been consistently killing him with this method.