Just sayn don't rule it out. And I won't drop it cuz it seems likely in order to get something special it would need to be tough
Okay dont drop it but dont expect anyone who has played the game to attempt it, its not something anyone is going to do its simply too hard. Its seems "likely" in your head, im in the same situation i want klassic costumes unlocked and if i thought there was even a remote chance that double flawless on expert on the pit with santa would unlock them i would be their constantly trying it , however it simply dosnt weigh up.
We have 4 hidden Kombatants and 10 (or 12) classic costumes however classic smokes requirements for fighting him dont change depending on difficulty so he can be ruled out, classic noob is effectivly a random encounter so difficulty isnt an object here, jade is currently unknown and classic reptile requires a double flawless.
So lets assume difficulty = reward (like your implying) the only known character who depends on difficulty is reptile, now again for the sake of argument lets assume jade does aswell, that means 5-6 hidden costumes unlocked per character.
Thats not ever going to happen its not reasonable. Multiple unlocks for 1 task.
Now i assume your retort will be "yeah well this is super difficult so obviously their going to reward you big?" which isnt an assumption based even the smallest amount on logic. ~
If and this is a huge IF, the klassic costumes are unlockable i believe winning all 4 of these fights might play some role in unlocking them difficulty not being a factor *see above*, HOWEVER goro and kintaro have ladder endings so we cant dispute the fact they could play a role in it aswell.