You'll need to get Ming's Harp first. Enter the White Boa and proceed to the ship's Guest Area then eventually to the Queen's Chamber. Once you have the Harp, wait until you have access to Numara Atoll. Go there to start a scene and summon a beast.

This battle isnt hard. Just equip Anti-Sleep or Ailment Resistance skills. Beast-Killers and Earth effect rings are your best bet on winning this battle. Forceus can also deal massive amount of damage to the enemy. Cast the usual healing and support spells first. Having Null All Elements skill equipped is also essential for blocking its All-Groundus and All-Aquaus attacks.

After the battle you will obtain the 1000 Year Memories accessory. This exceptional item allows your immortals to obtain +10 more skill slots, which is stackable from the existing +3 and +5 skill slot skills.