Ancient Wyvern is an optional boss in Dark Souls 3. You find him in the Archdragon Peak secret area. As soon as the fight starts, run up to him and make a left. Go all the way up the stairs and follow the path around until you are above him. You now need to wait until his head is below you, then do a jumping attack off the edge. You’ll land on his head and one shot him. Easy as that.