The Avengers and X-Men are on separate missions to rescue/assassinate the Scarlet Witch (who is in a coma and cannot partake in the battle). The Avengers and X-Men eventually clash and both sides aim to take each other out.
First round- fight until one team is KO'd or submits; morals on.
Second round- fight to the death, no morals.
Avengers: Captain America, Hercules, Black Knight, Vision, Sersi, Crystal and Thor
X-Men: Rachel Grey, Wolverine, Storm, Colossnaut, Nightcrawler and Rogue (Ms. Marvel powers)