Good pic, although I don't know why, but I think The Chosen Undead doesn't get enough credit, I mean, yeah he dies, but death is near meaningless to him, since he can just get back up again, and canonically, he DOES triumph over everyone, even Artorias, Manus, and Gwyn, weakened as the last one is.
While it is undeniably praiseworthy that he killed Manus and the Lord of Sunlight's husk, Artorias was pretty disadvantaged when he was put down. I mean, he *was* wielding his sword in his off hand after all. Don't get me started on his shield (or lack thereof)
I think the game outright says Manus defeated Artorias. Dusk of Oolacile IIRC says that Artorias lost because he wasn't human like the Chosen Undead because something something Humans are Special or something, I dunno, the game is vague as hell bout it.
Its because the Abyss is born from Humanity. You will not be corrupted by the Abyss, technically you are a part of the Abyss. Artorias is a "lesser god", same race as Orstein, Havel and black/silver knight. Just BE in the Abyss could corrupt Artorias, even with his ring. That's why he had his shield. But to protect Sif, he gave up the shield and the Abyss started to corrupt him. So against Magnus, without shield, his left arm broken and devoured by the Abyss... He didnt have a chance.
And no, he really did not fight you with all his strength. He would be the fucking hardest boss in the entire game. Imagine this boss, with an unbreakable shield, more damage and even more fast movement !