Dark Souls Remastered | Gwyn, Lord of Cinder solo on NG - To Link the Fire ending (and achievement)

You'll see a couple areas with rocks. Let's just call them pillars. A lot of Gwyn's attacks are horizontal. When you kite him around one of these pillars, he'll go toward you, elevating himself on one of these pillars. He'll then have higher ground on you and his horizontal sword swings more likely to miss you.