Yeah, the Sinner's Rise bonfire is kinda bullshit. I don't know why they arranged that one the way they did.
Lost Sinner, on the other hand, is much like Artorias from Dark Souls' DLC: if you're trying to block/circle around him, you're kinda doing it wrong. You CAN do it with a greatshield, or just a really high stability standard shield, but it's the slow, choppy way to do it. His sword is so long and his swings are so fast that trying to block and then attack through the combo will usually get you punished or just whiffing the attack (unless, again, greatshield to be right in his face the whole time), and dodging away is unlikely to get you an opening to attack. You have to man up and dodge into his attacks to get to his back.
Also, lighting the torches in his chamber apparently makes the fight a lot easier. This is what I'm told, at least; I've never bothered lighting them. You can, though, since you went through the belfry first so you should have the Bastille key to open the doors to light the lamps. They're up those stairs on either side of the boss door. Need a lit torch, though, which means either carrying a lit torch down there from the bonfire or using a Flame Butterfly to ignite your torch once you're there.