This boss can only be activated with possession of Ming's Harp, which is found on the White Boa.
To activate the Boss, head to the far North of Numara Atoll ( the rocky bit ), where a short cutscene will trigger and the fight will inititate.
Ming is a mandatory character in this battle, so it helps if she is levelled up. Before the battle get the Quad-Element Amulet from the Kelolon Village Royal Seal and skill link it to all your immortals. You should also teach them anti-sleep, to negate the effect of the Holy Beast's All-Sleep spell. Try creating some Level 3 Beast and Ground-based rings, as these are very effective when equipped to Kaim and Seth. To defeat the Holy Beast, just have Kaim and Seth attack or use combo skills (you can increase damage with Powerus magic) and have Sarah, Ming and Jansen use Ground-based magic on the beast. If the Holy Beast keeps killing the party with Hopelessness spell, try skill linking Persistence from Tolten so the immortals retain 1HP.