If I remmber correctly, they had to use nukes, a crap load of them against the black beast. The thing was, the black beast was unaffected by the nukes.
Prettty much, it took Hakumen, Platinum the Trinity, Valkenhyne, Terumi (Hazama), Nine, Jubei and Ragna the Blood Edge to fight the black beast, Ragna sacrificing himself to stop the Black beast movement
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Guilty Gear - (Main) - Baiken (Sub) - A.B.A - Dizzy - Anji Mito - Zappa - Sol Badguy
Blazblue - (Main) - Rachel alucard (Subs) Platinum the Trinity - Hakumen