Cosmic Weapon.

The move of the amalgam to create a single large move was an opportunity to end things quickly. That's what Legion thinks. So they forced the golden domain to shrink into their hand.

It compressed into a changing mass of cosmic force in their hands. First, they turned it into a golden whip which they lashed at the large limb. The golden whip cut through the black limb cleanly and completely. 

The black ball of flesh cried out in pain again. It hurriedly absorbed the cut tentacles to replenish its strength, even as they were disintegrating. But Legion wasn't going to give it a moment of respite.

They swung the golden whip about like a cleaver. Where it passed was marked by the distinct signs of the disintegration of space and time. They used it to prune the black ball of flesh. It stopped being a giant after having a large proportion of its existence shaved off.