I think the hardest boss in MB is smogger. There is a secret to smogger - Kill his minion first then him. After that, he will appear again and you can fight him and 2nd time ( His minion won't be there).
I think the hardest boss in Marleybone is... MEOWIARTY! Eh, him, 2,500 HP, a Wraith, 1,600 (I think) and a Clockwizard (450) HP. Lucky Malistaire didn't join the battle.