The Holy Beast can be found on disc 4, once you've acquired the White Boa. Go inside the Guest Area floor of the White Boa and head straight into the Queen's Room at the end of the floor. Talk to Ming who is standing there then press A on the Harp in front of her. You may need to visit Numara Atoll first, which is a small circular island on top of Numara to trigger a scene with Ming.
Suggested average level is 45-50+
This battle is straightforward. Just equip Anti-Sleep on everyone since the Holy Beast casts Sleep on everyone and performs a powerful spell at 9999 damage when it's HP is in critical level. Equip Earth rings on Kaim and Seth and cast Powerus on both, or simply cast Groundus spells with Sarah and Ming. When he casts Sleep on everyone, you have 3 turns to defeat him or he'll do the 9999 damage spell and kill, so take him out fast.