Persona 5 Royal

This achievement ties to a pair of boss fights emanating from the very bottom of Mementos, taking place automatically in the main story on 12/24 after you reject the fake Igor's proposal to return the world to its prior state (where accepting such an offer would give you the bad ending of the game). After retrieving your teammates from their jail cells in the halls outside the Velvet Room, and then traverse the Path of Qliphoth above the streets of Shibuya, you'll have one chance to heal up, put on your best equipment and accessories, and save, before you proceed to confront the first boss, the Holy Grail.

You'll know from what you discovered in your first encounter with Holy Grail is its healing spell, the Will of the People, ensuring full recovery of HP no matter how heavy you throw your attacks with the strongest attack buffs in effect. This time though, Futaba/Oracle will notice the red piping strands resembling blood vessels above the Holy Grail's cup and correctly determine them as the supply line for its notorious healing spell, and that cutting this off should be the first priority. You'll then get the prompt whether to send someone to take out the supply line, press and choose anyone that doesn't have a healing spell in their moveset, which can be either Ryuji or Yusuke. Like your battle with Kamoshida in knocking off his crown, you will need three turns, filled with constant attacks, in order to keep the Holy Grail distracted and unable to notice your party member. Don't waste any HP or SP on special attacks just to ensure they won't go for naught if it calls for Will of the People again. Just use basic attacks until the party member assigned to cut off the supply line does their thing. Once that's done, buff up your attack stats for your party be it through items like Rasetsu Ofuda or any party members or Personas with Matarukaja, and put out your strongest moves, as there's no resistances whatsoever for Holy Grail. As far as attacks from the Holy Grail goes, it'll have four Colossal damage scale Almighty type attacks. If you've leveled up your party high enough (around say, Lvl. 80-85 at least), and have strong enough armor, you can budget taking damage for a couple of turns on any party member(s) before you should consider healing. It's the Eternal Light attack you really need to brace for, which thankfully comes only after the Holy Grail calls for the Gathering Light charge move, giving you a turn to have your party members heal+guard. Follow with this attack pattern until you defeat the Holy Grail.

Of course, you're only at the halfway point of this epic battle. You'll soon have a cutscene where the floors and walls around Mementos open up, and a mammoth demon emerges from beneath, with the Holy Grail transforming into a halo now above the head of what is the final boss of the original version of Persona 5: Yaldabaoth.

When your battle starts, you'll note whatever you had for leftover HP+SP after beating Holy Grail carries over here, no healing beforehand. So if Joker and/or anyone else is low in the HP department, prioritize healing+buffing in your initial turn. Intimidating as he looks, Yaldabaoth will only have one turn to make his own move (Arrow of Light [Heavy Almighty-type to one ally], Dekaja, Dekunda) ...it's when he calls forth his set of four arms representing the Deadly Sins you should be wary of. Each arm has its own unique set of affinities for which it repels attacks, and has their own turn to choose what to use on you, be it attack spells or spells that inflict certain special ailments or adjustments such as making spells requiring SP more expensive. I've listed down the description for each arm and their own repel affinities and summarized movesets:

Gun of Execution - Repels Gun and Wind. Uses Distorted Lust and Distorted Avarice to heavily damage one ally and afflict Lust and Hunger respectively for one turn. Uses Heavy to Colossal Gun attacks.

Bell of Declaration - Repels Fire and Psy. Uses Distorted Vanity to inflict weakness to one ally and Distorted Envy to inflict Jealousy on one ally. Both spells last one turn. Other moves include Gospel (Colossal-scale Almighty attack to one ally) and defense-related all-party spells buffing and debuffing for three turns (Tough Law and Frail Law respectively)

Sword of Conviction - Repels Physical, Gun, and Electric. Uses Distorted Gluttony (Raises the cost for using skills by double its value for one turn, all allies). Also uses Sword of Judgment (Heavy Physical damage, all allies) and Wind Cutter (Colossal Physical damage to one ally)

Book of Commandments - Repels Ice and Nuke. Can inflict status ailments (Distorted Wrath, raising attack but reducing defense to one ally) Set up an ally to lose SP (First using Distorted Pride, followed up with Divine Punishment if afflicted party member attacks Yaldabaoth). Other attacks include a one move each for each type of affinity except Almighty.

As you can see, Yaldabaoth and the four arms hold NO resistance to two affinities: Bless and Curse. That can be reserved for Joker and his Personas that have All-foe non-instakill moves of these types in their movesets. The main focus is attacking Yaldabaoth himself as draining his HP heart will end this fight, while he can continuously revive each of his four arms (at 50% of their HP after you defeat them in their first goaround). The more arms he has alive, the more turns he has to afflict+attack your party, so be sure to take down as many arms whenever you're able, mindful of their own unique repel affinities. Keep up with this pattern until you get his HP heart to 90% or so empty, and he will bring forth all of his arms out at full health, and call for Divine Apex, the charging setup move for his strongest move, Rays of Control. The bad news is the potential it holds of wiping your party out. The good news, on the other hand, is that Divine Apex requires TWO turns for a full offensive charge. Use the first turn Yaldabaoth is charging to buff attack for all allies and knock out the remaining health he has left. Even if it doesn't happen after the first turn, it'll definitely happen in the second (heal+guard if you don't feel it's worth the risk). You'll know you've reached the end when Yaldabaoth raises his arms up and shifts his upper body back after you land the last attack on him.

By this point, you'll have a cutscene where Yaldabaoth is still standing, launching desperate maneuvers to bring you and your teammates down. He'll taunt you constantly about humans mocking you for resisting gods like himself, but a defiant Morgana will say otherwise, still voicing his belief the Phantom Thieves can prevail as long as the will is still there. The crowd in Shibuya catch note of Morgana's voice slowly getting louder as chaos ensues and people disappear, and soon they see the Phantom Thieves symbol on the giant TV screens, voicing their support for the group to bring an end to this madness, and further aided by your NPC Confidants. The Thieves soon get up, renewed in motivation by the cheering crowd beneath them and Morgana, ready to bring down Yaldabaoth once and for all.

Here, Joker will catch wind of the percentage of people believing in the existence of the Phantom Thieves as measured on the Phan-Site hit 100%, and his Arsene Persona will soon evolve into Satanael. Now, you get one more round with Yaldabaoth, but it'll amount to no more than a proverbial victory lap, as his Rays of Control will be completely nullified. Joker, on the other hand, will have only the Sinful Shell attack available, but that will be all he'll need to literally put one through Yaldabaoth's head, and bring your journey as leader of the Phantom Thieves to a close (if you were playing OG Persona 5, or hadn't maxed out your Councilor Confidant aka Maruki before 11/18 here. Otherwise your game continues into the New Year and Third Semester!)

Your achievement will pop on screen in the ensuing cutscenes where the Treasure from Mementos materializes in the form of the Holy Grail in much smaller and less grandiose form, and Morgana thanks you and the rest of the Phantom Thieves on a job well done, before he soon disappears.