Beating the Redscale Apocalypse Dragon without being in New Game Plus. I hex edited my save to unlock the secret boss in my first playthrough, as well as setting the difficulty to Regicide along with it. Killing the dragon in one hit is totally possible but requires a lot of gambling. One shotting requires using Strohl's Royal Warrior with the correct passives + 3 Samurai teammates, on top of Strohl having to land a crit with high damage roll, which is a big gamble. In my case, I'd just use the phys-based Royal Archetypes and 2 shot it as that doesn't require as much gambling and is a quite consistent strat. Having 3 damage dealers in this comp is really great, if either Strohl or the protagonist fail to land a crit, we still have Basilio to finish it off as a backup.
P.S. In New Game Plus, you can obtain another Overlord's Sash accessory (for triple MP cost, Synthesises deal double the damage) for Basilio which is a massive upgrade over the Herald's Banner (front row dmg x1.2). And one more thing, the Prince could deal about 20% more damage if he had the Dragon's Fall sword (all skills damage against dragons x1.5) equipped over the King's Sovereignty. This is due to the latter sword's passive not contributing anything to despite its higher base atk.
Chapters
00:00 Intro
00:24 The Fight
01:45 Rewards
03:12 Party Builds