So I'm fumbling my way through the catacombs. I know the skeletons can only be killed for good after you kill the sorcerer that's usually near them. My problem is I can't find the last sorcerer that's keeping the last batch of skeletons alive. Where is he?

Also, is there a safe way to deal with those skeletons in wheels in the valley just before the boss? Is there another bonfire I'm missing?

Can't help you with the last sorcerer, but the skeletons stay dead if you use a divine weapon (upgrades from +5 weapon, and you need to beat the butterfly boss I think).

I found him. Inside the tomb with the lightning-shooting statue, follow the way to the boss through the broken wall on your right, go down the ladder and then, instead of going down through the broken floor, look for another broken wall in the dark and you'll find him guarded by two skeleton archers.

Also, there's a second bonfire behind a fake wall near the ladder you go up to remove the spikes from the final bridge.

Also, there's a second bonfire behind a fake wall near the ladder you go up to remove the spikes from the final bridge.