The chariot skeleton thing. Figured out what to do fairly quickly, but I ran out of stamina so fast whilst waiting for the chariot to wipe out the other guys and ended up getting my ass handed to me. Did it first try today, of course. Always the way.
And there's another boss that I'm nowhere near able to beat, the big thing made out of bodies in the Black Gulch. Don't think I'm meant to be there yet - although it's great for farming titanite shards and large titanite shards, and I think a rare drop of titanite slabs as well.