My favorite boss indeed! After Sif, Priscilla and Quelaag, he reminds me the fight against Allant, but Gwyn has no mercy in close combat since Allant he attacks with some powers and his swords but, I don't know who is the most lethal of two.
At the end of it all, after all that fighting and struggling, there is just a sad, lonely old man. Not really even a great evil warlord, just, a lonely old man...
He is a great warlord god, but he is way past his prime and is now just a husk that waits the chosen undead in the Kiln. I think he WANTS you to beat him since everyone says that it was his wish for the flames to be linked. Could he be testing the chosen undead by putting his own life on the line to make sure the flames are linked?