Jotun of Grief is the final guardian encountered in Tartarus. It's Arcana is the Hanged Man and is located on 252F in the sixth block, Adamah. Upon defeating this boss, the Judgement Social Link will reach Rank 9. Tartarus will remain blocked at 254F until the game's (playable) final day.
Jotun of Grief is designed to exploit an unprepared party, as it absorbs all attacks other than Pierce and Almighty. If any party member uses a default Physical attack that isn't Pierce-based, the enemy's strategy of Stagnant Air + Infuriate would have that enraged party member attacking (healing) it twice.
It's recommended to bring three allies that use Pierce attacks; Yukari Takeba, Ken Amada and Aigis. The main character should use a Spear or Bow weapon with +Rage Resist or equip a Persona with Null Rage. Allowing other party members to become Enraged can be a double-edged sword; damage output is increased and they get to attack twice per turn, but their defense stat will be significantly lower. Therefore, the protagonist should be immune to Rage in order to heal and revive party members. Buffs and debuffs are also particularly helpful here since the boss can use any of the three "Ma-" debuff moves.
Upon reaching low health, Jotun of Grief radically changes its strategy by casting Mind Charge and Megidolaon continuously until it is killed or healed above the critical threshold. Note that it will also no longer cast Infuriate during this phase.
Alternatively, it is possible and advisable to fight it using any character with a "break" type spell, followed up by the according "dyne" spell. Also bring along a character who is proficient in buff and debuffs and any method necessary to counter the boss enraging spell, may that be item or curing spell.
The Jotun of Grief appears on Paths 5 and 6 of Yomotsu Hirasaka within the Midnight Channel. It will attack with Wind skills and replenish its HP with Life Leech. After being defeated, it may drop a Ripper Horse. Selling 10 Ripper Horse at the TV extension of Daidara will unlock the Futsuno Mitama sword for the protagonist.
In Golden, it will attack with God's Hand and Megido, and occasionally buff itself and allies with Revolution.