Master Fortress is the exact same every single time, and the only RNG that will occur in it is when enemies do any actions past their first attack, which if you handle it right they won't ever do. As someone who no damaged 9.0 Master Core, the Fortress is the easiest phase by far after you learn it and how to handle its enemies; it's just a simple short platforming level with simple-to-handle enemies, and after some practice I became able to no damage it nearly every time. I also think Master Core's difficulty in general is overrated by people, like both hands together in Brawl were a lot harder while solo Brawl Master Hand, Duon, and Tabuu were comparable or more difficult, and the only thing that made no damaging 9.0 Master Core take a while is that I would have to play through Wii U's Classic Mode every time for a single try (since dying and continuing forcefully downgrades the difficulty); in the actual amount of time I put in just fighting Master Core it didn't take that long to do for the first time at all.
But yeah **** the duplicate custom moves and Mii outfits, being able to get duplicates of them that did absolutely nothing was absolutely terrible game design that only served to make getting them a ton more tedious, the Mii outfits are what have prevented me from 100%ing both versions of Smash 4, as I don't have much desire to mindlessly grind out hours on end for a slim chance to get one of the couple remaining Mii hats I don't have. Getting all the stickers in Brawl was also an extremely tedious grind, but at least duplicate stickers there served some sort of ingame purpose (as stickers were powerups in the SSE and each sticker could only be used once).
