I've recently been grinding my skills in the game but not in the whole, just individual bosses (MG Ray, Monsoon, Sam & Excelsus specifically) and I would like there to be an category for single boss fights, so people can show their efforts to others against bosses they're good at, and not the ones they aren't

If i'm not wrong you mean IL? There are already those, you can check all the Leaderboards, Bossrush% is also a thing but if you want to do each boss individualy you still can

Yeah see individual levels for individual boss fights