Welp, I don't know if anyone will ever need that, but here is rules for Final Boss:
1 stage: Actually, the only stage I didn't get how to play, I just had large amount of HP, so it wasn't a problem, looks like it requires to guess, which one of his copies is real and attack it.
2 stage: Very quick chase-segment, literally one long room
3 stage: Need to kill and destroy spawning enemies(Kill with Hero Sword and Destroy with Phantom Sword), they will try to break four columns of elevator and will spawn every few seconds without waiting for previous ones to die, so the faster you destroy previous ones, the easier you will proceed. Also, I recommend to open Spawning Chest before you use all of your magic, mobs might be annoying while you can't fight back so might want to have spare potion to use(Thankfully, number of enemies are finite and if you'll lose here - you will continue here as well after Game Over scene)
4 stage: Boss will summon Swords so you need to avoid them while aiming at standing in one position Boss, if you have more HP, you can just let these Swords hit you while you standing near Boss so you can hit swords and send them back.
5 stage: Relax and finish this mf.
Also, after clear, game will remain as if you didn't defeat Final Boss, but CG-Gallery(Not H-content though, scenes from story and standing heroine), but you will need to complete every Hidden Level at every difficulty to gain every scene(1 scene per level/difficulty, also you'll need to finish this game three times from the very beginning and get S-rank for every Normal level three times as well, and here is a big chance that some levels might be broken and you will get S-rank, but it will not be registered), so you don't really need to defeat Final Boss to get H-scenes, only if you want to see ending or have feeling accomplishment.
If you really liked the game this much, you can try to open everything, but my noob ass had enough even at the easiest difficulty so I will take the win, watch Game Over scenes few more times and delete this game~
P.S.: Also, here is Save files(Same ones as previous, but with bought HP and Invisibility bonuses for someone who wants to play more freely)