What you described is normal with this encounter. There is a bug where his stats (hp, skill ranks, etc.) will revert to his human form's if you save/load at any point after he transforms.
His "weakened" status only affects his human form stats. Which means it's easier for you to cheese him by killing him without letting him transform. Once he transforms, his stats will be exactly the same whether he's weakened or not.
Yes, Lone Wolf pretty much destroys this boss, even when he's in his demon form. Dual Lone Wolf is probably a bit overkill. I guess in your 3rd playthrough you got to go first and killed him (or disabled then killed him) before he transformed.
