So, the Blue and Red cosmic brothers from the DC vs. Marvel crossover change their minds about coexisting at peace within the Omniverse and start a new war between both universes. They manipulate the minds of both Zeus´ so that they believe that the other group of Olympians want to invade their realms and become the ONLY remaining Olympians.
So all Olympians of both universes in character, and bloodlusted as only an ancient deity can... . No prep time. They´re all armed with all their standard weaponry, as well as the standard powers of that particular universe Olympian. As each pantheon believes that their very survival depends on achieving victory, the battle is to the end. Both Hercules´are present (but no Wonder Woman). However, the Brothers have restored the Marvel Hercules to his regular godly levels of power.
The Brothers transport Marvel´s Olympian pantheon to the DCU, where the fight takes place on Themyscira. But the Amazons are a non-factor, as the Brothers have teleported all of them elsewhere... .