Rewards for not taking damage from a boss?

Galamoth Essentially. Say every boss drops a medal like in Ecclesia and you get the "Ultimate Warrior" achievement for getting every single boss medal. Its the best kind of "look at me I did it" kind of reward that hardcore players can go for but doesn't have some kind of powerful spell reward that deprives more casual players of more impactful content that most players would likely never be able to pull off. 

If done right, maybe this could work.
But I would like it as just a list separate from the game itself.

When a game basically ends up with "you finished... but not really" feeling.
But if they go with a Metal Slug approach, I will have no problems with it.
You can have the list of self-imposed challenges and such in a separate place.

In Metal Slug you don't need to finish levels saving every prisoner and not die.
But if you do, there's a little list of these things that you will find for each level.

It could be like that, and also in Rockman 9 (or 10), there are those challenges:
Also separate and don't add to the gameplay itself, but they're there. The usual speedrun type of thing, and the self-imposed challenges, such as not dying, or not taking damage, and so on.

I could imagine Bloodstained having this and maybe a ranking of sorts.
It would be nice because it could add a little arcade feel with these challenges, while still not affecting the gameplay itself, and maybe even add more replayability for people who like having a nice record.