Persona 5 Royal

This achievement ties to the boss of the Casino Palace, the cognitive form of Sae Niijima. She stands out from the bosses of the previous Palaces in that her strategy relies on an enlarged Roulette wheel, rigged to ensure every outcome falls in her favor. You must find the source of the cheating aid to destroy, and then make use of the openings to attack Sae as she'll use the wheel to give her a temporary affinity alignment as added defense from all attacks from other affinities besides the one holding superiority over the affinity she's aligned with.

To prepare, you should have your party on average at least at level 60 (or even level 50). But the motto of 'Take Your Time' on the load screen when you first boot up the game is a slogan worth following here. Grind for money and XP for several hours until you get your party to your desired target level, if you maxed out your Moon Confidant rank you'll get to this target faster. It's also a good idea to start boosting the rank of your Strength Confidants to at least Rank 5, as it will allow you to fuse and create Personas at levels higher than the current level for Joker, giving you everything from stronger attacks to potent stat buff spells that can cover your entire party, including ones that automatically kick-in when you begin battle. For who you choose to pair with Joker, make sure to have a healer with a spell to heal your entire party. Makoto with her Mediarahan should fit the mold. The other two slots can be filled any way you choose, but Akechi is a great suggestion, as he holds a great array of Curse and Bless attacks to compliment an all-foe targeting Megidoalon Almighty spell and a Megaton Raid single foe attack with a good chance of incurring Critical damage. Also note of the special items you've gained as a result of getting all the Will Seeds in each of the previous five Palaces and then bringing the crystallized clusters to Jose in Mementos for conversion. You should have the Ring of Lust, Ring of Gluttony, and Ring of Wrath available in your equipment inventory to apply to your party members in the Accessory section. Each of these accessories will amp up your attacks in some shape or form, and will be a big help here when you destroy Sae's cheating aid and can then focus on attacking her.

When the battle begins, Sae will start the Roulette wheel. DO NOT ATTACK WHILE THE WHEEL IS SPINNING. She will correspond with a counterattack that lowers your HP down to just 1. She will spin the wheel again and again at random over the course of the fight, so just use these opportunities to simply defend and/or use special stat buff items/spells for your party. After your first turn, Sae will call for bets on where the ball will land, No matter if you bet high stakes, or low stakes, you'll lose in the end, where the penalty is loss of half of HP. After the result, Morgana will ask if you figured anything out about the outcome. Select "I most certainly have", then select "There's a glass lid." Morgana will then see the hint about the ball floating for a split second on the number you picked before popping onto a losing number/color. You'll then get a prompt of when it's time to send a party member to expose Sae's dirty trick, as in shoot down the glass lid the ball is intent to land on. Press for Yes, and pick whoever you want to jump outside of battle and mount a vantage point over the wheel. Akechi is well recommended for this task, so go with him. Use your ensuing turn as the wheel will get spinning again to heal your party/stat buff/guard. Afterwards you'll get the prompt to bet again, this time, choose the high stakes route, since the party member you picked to fire off at the glass lid will be set to execute their move and you'll win your bet this time around. (Which should be 9,000 yen) After this exposure of cheating, Sae will metamorphosize into a Leviathan form of herself, with a monster gun for one arm and a giant sword for the other. In the first turn with Sae in this second form, she'll spin the wheel again, only this time where the ball lands is what the elemental affinity she will associate with, and where she can resist spells from all other elements except for the opposite element of the one she temporarily associates.

The chart I'll put below lists the spells and their rival opposites. Whatever affinity the roulette ball should happen to fall on, use attacks from the affinity that's listed either left or right of the affinity Sae is aligned with. For example, if the roulette ball lands on Curse, you should use Bless attacks, and vice versa if the ball lands on Bless.

Wind - Zio
Fire - Ice
Psy - Nuke
Bless - Curse

Make note that you can use of elemental attack items in your inventory if your character doesn't have any moves that are exact elemental opposites of the element Sae holds at the time. This creates important All-Out Attack opportunities that can net significant damage. Just remember: Stat buff when the wheel is spinning, attack when the wheel stops. When you get her HP heart down to under 50% or so, the next time she spins the Roulette wheel, the ball will land on Almighty, making all attack moves being of that type, but in the process opening herself up to taking normal scale damage from all other element skills. If you've played your stat buff strategy right, offense is your best defense here in finishing her off, and it's all systems go for each of your party member's strongest moves, and you'll then prevail like the real life Sae does as a prosecutor in her cases, at a 99% success rate!

The achievement, like all the other Palace related achievements in the game, will pop not after immediately defeating the boss, but instead when you wind up in the interrogation room with Sae on 11/20 after a brief cutscene in the Velvet Room.