Okay so if you are wondering how do you even get the keys you would have to battle certain character in said paths and then you would have to abide by the given Bonus Condition in order to get the key or whatever else is on the bonus, the easiest and fastest one i have found was in Gyomei Himejima and Tengen Uzui Path on the intermediate level, Where they both have Sanemi Shinazugawa as the person you need to beat right at the beginning and all you need to do is battle him without getting hit and keep in mind he is very very easy to beat, and once you do beat him u dont even have to finish the entire path, you can just leave the training and do it again, but ofcoruse that would mean you doing the same battle over and over again until you get the amount of keys you would need. and thats all you need to know.

Have fun!!
https://docs.google.com/spreadsheets/d/1IgJ6TuNqEgVBFwK0hU1zp6jRaUlq8Nh4Vl8Npvp-CKY/edit?usp=sharing