The boss of Junessic Land is King Frost, a powerful ice-elemental Persona. King Frost is weak to Fire attacks, so ensure you have characters with strong Fire skills (e.g., Agilao, Maragi) and Fire-oriented Sub-Personas. He primarily uses Ice attacks, including 'Bufudyne' and 'Mabufudyne', which can deal significant damage to your entire party. Characters with a weakness to Ice should be protected with Ice Wall or by equipping Sub-Personas that nullify or resist Ice.
King Frost will also summon 'Jack Frost' minions. These minions are weak to Fire and Electric. Prioritize taking them out quickly to prevent them from boosting King Frost's actions and to gain Boosts for your own party. King Frost has a high chance to inflict 'Freeze' status, which can incapacitate your characters. Have items or skills that can cure Freeze (e.g., Patra, Amrita Soda) ready. Focus on exploiting his Fire weakness, maintaining buffs/debuffs, and managing status ailments to secure victory against this chilly foe.