I beat True Masakado at Level 99. Unfortunately, because I didn't remember that the Switch can't take videos of SMT V:V in the moment, the only proof of this I can offer are screenshots of my team and a description of my strategy¹.
I'll start with the team's attackers.
They're here to use Omagatoki: Dance, Fickle Personality, Pandemonic Feast, and multi-hit attacks to dole out damage as fast as possible. Vritra was on the opening lineup because he had space for Resist Phys, while Cherub waited in the stock until Vritra had used up Endure and Enduring Soul so that he wouldn't have to deal with Brave Blade. Cherub being built to attack with two different elements is half because he was built that way before I started trying to defeat Masakado, and half to get a bit more defensive leeway near the end of the fight².
Now, you may be thinking "there's no way this team could survive Masakado's crazy-ass damage output" , and you're absolutely right. That's where the support members of the team come in.
This team's defensive strategy is centered around how Masakado does not have a special reaction to Makarakarn; he treats the repels it creates the same as any other. Between how I need to spend some turns keeping buffs and debuffs up and how Masakado will replace skills that were negated with Freikugel for a few turns³, Makarakarn can't protect me forever, but it, Endure, Enduring Soul, and a Whittled Goat could buy me just enough time to put my offensive strategy into action. Dís and Norn would still need to know Makarakarn even if I wasn't refusing to use Knowledge of Tools⁴ because I needed to use the skill more than 10 times; I went through 6 out of 10 Magic Mirrors in the winning attempt.
On Omagatoki turns, the Nahobino uses Lunation Flux while Dís and Norn switch places, giving all their turns to the attackers to minimize the amount of time they need to buy. When circumstances forced me to give up an attackers' turn to do something important, I'd do my best to make Vritra or Cherub take the fall because Yurlungur was the best attacker. Evergreen Dance was theoretically meant to lower the amount of time between attacking turns, but I never had time to use it, while Revival Chant was used to revive Yurlungur, Dís, and Norn because they did not have understudies.
Finally, we have the team's odd man out.
The only skills that matter here are Magatushi Thriftiness, Fairy Banquet, Fierce Roar, Resist Force, and Enduring Soul. Boon Boost was supposed to matter, but I didn't see that it wouldn't do anything.
Nahobeeho had two roles during the fight. His first one was being in the fourth slot of the starting lineup after Yurlungur, the Nahobino, and Vritra, allowing the Nahobino to use Fairy Banquet on the first turn of the battle to get buffs up ASAP. Afterwards, I'd summon Dís in the fourth slot so she could start applying debuffs. His second role was to have Fierce Roar and drain an element I wasn't using offensively; when I was pushed to the brink of disaster, he could come in, grab Masakado's attention, and drain Ice Dracostrike to buy just a little more time.
My defensive strategy bought me exactly enough time to win with my offensive strategy and not a second more. On my final set of turns, the only thing I could do to stop him from vaporizing me with an Omagatoki: Critical-boosted Recalcitrant Execution was to kill him first. I stopped to calculate whether using Luster Candy to buff the recently-resurrected Yurlungur was worth it; after counting out the remaining turns and seeing that the opportunity cost was Cherub's turn, I decided use it. Since I haven't run the numbers⁵, I'm not sure whether Luster Candy was actually worth the turn I spent on it, or if I should have unsummoned Cherub for a proper Buckgetter Special; all I can be sure of is that Yurlungur brought me just past the finish line.
Footnotes:
1. Even this wasn't trivial to obtain. In order to show the build the Nahobino had during the fight, I had to splice a screenshot of my skillset taken after my victory together with a screenshot of my stats from the backup save I made in case I regretted using a hard-to-reacquire essence.
2. When hit with non-almighty damage, Masakado will replace attacks of that element with Diarama on his next turn, and this takes priority over replacing a negated skill with Freikugel. If I know he's going to use Freikugel in place of Wind Dracostrike or White Dracostrike, I can have Cherub use Floral Gust or Judgment Light to stop him.
3. To be specific, Masakado will replace the negated skill with Freikugel for n + 1 turns, where n is the number of times he's gathered magatsuhi.
4. When I couldn't find Knowledge of Tools in Minato, I thought it got removed from the Vengeance for being overpowered, leading me to not use it even after learning that it only had its availability nerfed.
5. For anybody who wishes to calculate what the right move was (which may include my future self), at the start of my final turn, I had 4 press turn icons, Yurlungur had no buffs or debuffs, and Cherub had +2 to everything.
