Record of Ragnarok

Every thousand years, all the gods gather in Valhalla for an assembly presided over by Zeus, the supreme Greek god, to decide the fate of mankind.
That year, it was decided almost unanimously to destroy mankind. But the Valkyrie Brunehilde interrupts the conference and proposes that the gods compete against mortals in the Ragnarök tournament, to determine whether they are really worth annihilating.
The tournament will pit 13 gods from various mythologies against 13 powerful humans who have made history. If humanity wins at least 7 of the 13 duels, then the gods will spare humanity.

Thirteen Valkyrie sisters merge with 13 mortal combatants to ensure a fair fight.