In general, think of it this way: Because you are the dev and know all the mechanics and secrets, unless you have no idea how to play games beyond a casual level, you are automatically going to do better than the vast majority of your players. You might get a small percent that are better than you. In general, you want to slowly train the players so that they can adjust.
Think of it like how rhythm games usually work. You have different difficulty levels. For most players, they'll start at beginner on the easier songs, then slowly as they get used to the game, they'll work on harder songs and they'll try harder modes of the easier songs. With practice, they will eventually be able to (as long as their physical state allows) do the hardest modes on the hardest songs.
You can use that here as well. Have your bosses slowly ramp up and have different difficulty levels. For example, story mode, every enemy might be 'normal' difficulty. If the player fails a level enough times, it might even offer an 'easy' difficulty. When you beat the game, it might unlock the harder difficulties, but you can also have the harder difficulties in a boss rush mode, arcade mode, etc. On harder difficulties, the bosses might have additional movesets that are harder to deal with, there may be additional mechanics that the player has to learn, etc.