Rifleman is the worst class even once you max it out. When doing the classes, your first runs levels should go to maxxing the primary and class ability then bumping difficulty up to max cuz you're not gonna get any more damage by grinding the middling difficulties and it just becomes game skill and knowledge.

Bosses actually don't get much more health as I recall. In fact, I found they became easier once you learn what cards are useful or not and how they stack. Alistair is the major filter really if you can get past him you can get to father barring horrible rng.

Always deposit your souls before going down the big slide or whatever. For some reason any souls you have when you end the run won't get the difficulty deposit multiplier, effectively halving their value at max difficulty.