Ancient One-Piece is the sixth stage in Low Tide Beach.
2 Sir Seals spawn simultaneously at the beginning. About 25 seconds later, a Bore comes out. About 30 seconds after that, a Director Kurosawah spawns with two THE SLOTHs. No enemies are deployed for the rest of the battle.
Possibly the biggest threat in the battle is the Bore. The Bore comes out extremely quickly during the battle so put out attackers right away. Use anti-red status effect cats, especially Knockback.
If you have Dark Cat, he is a very effective meatshield against the Bore, as he has longer range and can actually deal damage as a meatshield.
Once you beat the Bore, Kurosawah and the Sloths are easy. Just use Jamieras and Dragons, and you will most likely win the battle. Needless to say, the higher the level of the cats, the better. If the attacks are synced (by sending out meatshields with some luck), a fast-moving attacker with high DPS like Legelan Pasalan or Li'l Valkyrie Dark can do a lot of damage to the slow-attacking SLOTHS and Kurosawah.
However, if you have A. Bahamut, you can easily kill the Bore and destroy the base before the sloths or Kurosawah can even come out.