Well i personally think the Master of Masters should be a lot tougher than Master Xehanort with the X-Blade and any other secret boss before. I also think that the fight should happen when Sora was saving Kairi, where the MOM appears before him and wants to challenge him to a fight to test him. The MOM should have 25 health bars, the largest amount ever.
In the first phase, the Master "only" fires the za spells and attacks with his Keyblade, which would be his original Keyblade rather than that of the one that had his eye on. However, he fights swiftly and deals a lot of damage so it isn't easy still. When he starts to lose health, by the time 5 of his health bars had been drained, he begins to use a technique where he throws his Keyblade and then instantly teleports so not only does his weapon hit you, he also gets to slash you. When he had lost 8 of his health bars, the Master becomes able to teleport and loves to do so, suddenly appearing and striking you. He also can change his Keyblade into multiple forms like a whip to attack from a distance and a glider to fly while attacking with large balls of ice, fire thunder or wind.
When the Master of Masters has lost 10.5 of his health bars, he starts using insane magic like Hakaishin Champa said, such as Ultima, which is a large blast of pure energy, Zettaflare, which fires a huge blast of heat, and raining down meteors. Along with that, he should also become able to channel magic into his sword for a big powerful attack, such as when he electrifies his Keyblade and then slashes it down for multiple blades of lightning to scatter, creates a large blade of water that when hitting the ground unleashes water all over, and the same applies to a huge flaming sword his Keyblade becomes while air magic sword scatters into multiple small blades. By the time he's lost 15 bars, he can start firing storms of thunder and wind along with also creating fiery vortexes and can also use a gravitational orb for a powerful explosive attack.
When he is pretty close to death, the Master of Masters begins getting serious and uses both light and darkness, unleashing large meteors of darkness and balls and slashes of light along with also empowering his Keyblade with either to slash you through the sky. He'll also fire large rays of either light or darkness and can use the same health-draining technique Xemnas did, only this time Sora has to press multiple buttons as quickly as possible and if he fails he dies. His Keyblade transform also becomes able to access graves and gauntlets where he attacks swiftly and with great power using either light or darkness. They are quite damaging. Once he has only 2 bars left, the Master will begin creating Heartless, Unversed and Nobodies when possible while he recovers health. The barrier he places will only lift if all the foes are killed. Along with that, while maintaining the shield, he will begin to fire storms of Keyblades like Vanitas did and not only that can fire clocks that slow you down to be hit by either a stream of lightning or multiple fireballs. Neither are weaker than the other.
Once Sora "wins", the MOM merely smiles and tells Sora to do what his heart tells him to do but he warns him that they will meet again and next time he won't show any restraint, clearly proving even after all that power and having exhausted Sora, he isn't fighting at full power yet before leaving via a Door of Light.
The brand new secret ending show the Master of Masters arriving in the Keyblade Graveyard to reunite with his students and he tells them their goal is to create a better world where light and darkness can coexist. The screen says "The Master of Masters will return as the main antagonist in next game."