Now you know my pain, DPB.
I did eventually
manage to kill Fume Knight (albeit with help from another player), but it took a ridiculous number of attempts, and many embarrassing co-op deaths.
The difficulty of the Ivory King DLC was far more reasonable, I even managed to kill the first boss on my first try. That said, I didn't bother completing the optional area there since it was such a chore to get through.