Wolnir is a perfect example of a boss with a strong appearance, but an easy reality. The boss fight is really dumbed down to just smacking the 3 glowing rings and once they are destroyed, the fight ends. Of course, he will fight back against you by slamming the ground every so often but it's not gonna stop players.
The easiest strategy is to destroy the ring on Wolnir's right hand (or your left when looking at him) first because it is the hand that will hang back the most later on which makes it harder to hit. After that, go for the two on the other hand and watch out for the poison cloud that deals dark damage.
Wolnir will then retreat into the chalice he came out of, allowing you to claim victory.