Big Ben is the last dungeon of Marleybone that needs to be cleared. From your message, it sounds like you attempted to solo it. For starters, I don't recommend doing that but if you are confident you can beat it, then go for it! The landmark has 15 floors, most of which has enemies you will have to beat. There are few instances in which you can heal up so I advise you take advantage of those. I believe the healing sprites respawn too. And if you're up for taking on the secret boss, just knock down the milk jugs three times and you'll face him soon enough. Good luck!
Due to this topic I joined a team up request for Big Ben the other day. They turned out to be farming and they ported in a friend of theirs toward the end, who did most of the hitting after that. But hey I tried and it was a fun diversion.