After the Ra-Seru heroes defeat the Delilas Family and blow up Koru with the TimeSpace Bombs, the highly wounded Gi, Lu, and Che merge with Koru and order it to self-destruct, planning to blow up everything between Nivora Ravine and Sol. The Ra-Seru heroes fight Koru in order to defeat it before the explosion comes.
Koru is a timed boss fought. Only four turns are available to take out Koru before it self-destructs and causes an automatic Game Over. Koru will attack three times before preparing to explode, and each attack models attacks from the Delilas Family: Gigaton Press, Blade Breath, and Thunder Needle. Each attack will be performed once and will hit a single target for between 500-750 damage. Though the attacks are weak, Koru will sometimes attack the same character repeatedly, and if a character gets knocked out it is almost a guaranteed Game Over unless a full Point Card is used since no turns can be wasted for healing. After doing three attacks it will power up with Ultra Charge and the very next turn self-destruct with Dead End Crisis.
Vahn and Gala should constantly summon their most powerful spells - ideally, Vahn should have Kemaro absorbed and Gala should have Aluru. Noa should constantly attack with her Noa's Ark, and it is a good idea to equip her with a Power Ring and a Mettle Gem before the fight in order to make her attacks pack the biggest punch. Attacks should be constant, whether through spells or physical attacks. If Koru cannot be defeated, return to the Fire Tunnel and grind before trying to fight Koru again. Additionally, the Point Card serves its purpose best in this fight.
If the Spikefish Flute has been obtained from the fishing hole in Sebucus, it can be used right after Koru does Ultra Charge. This will prevent Koru from exploding and resets its countdown to 255, easily enough time to finish off Koru with no trouble. As an added bonus, Koru will only attack with glitched versions of Thunder Needle and Blade Breath that have no element animations and do no damage.