Crimsonwood 2

How's this. Each stage has one main objective, which can be done regardless of class or with any of a large number of classes (i.e. requires any ranged attack), and several side objectives with more specific requirements. For each of these side objectives you gain some sort of key or pass. In order to advance to the boss stage, you need a certain number of passes, which is low enough that being able to pass is pretty much a given if you don't just have ten NLs. Then, if you have a certain higher number of passes, you get to access a harder boss fight instead and reap better rewards/higher chance of good rewards.