It depends on build, stategy and overall experience.

The first time I went up against the duo Ingot my face kicked in. But that was due to some poor build choices and my own odd luck as somehow I managed to get to that point way under geared.

The next time I faced them it was waaay easier. I knew what I was doing. Had a better more focused build AND a +10 Pyro Glove with GCFB nukes.

I think the problem here is that many of the rankings people make are after they have played and beaten the game a few times.

I would love to compare a list of boss ranks from a person who was fresh off their first playthrough and a grizzled series veteran.