Father's Back is the seventh stage in Sub-chapter 25, Scars of War.
Using stackable attackers is good here to take out the Gory Blacks. Watch out for Elizabeth because without treasures, her attack and shockwake are dangerous .Once the field is clear, send out A. Bahamut to deal huge damage to the Sunfish Jones. Repeat this until the sunfish is dead.
No-Gacha options: Bringing Bombercat helps a lot, as well as Dark Lazer to quickly take out gory blacks. The rest is the same as 1/2/3 star. Send A. Bahamut if the field is clear, repeat until the fish is dead.