Captain America vs. M.O.D.O.K.
Grab a friend to do an Xbox LIVE match. One person (first player) selects Captain America in their team while the other person (second player) selects M.O.D.O.K as one of their members, then play the match.
Can be obtained through Ranked, Player, and/or Private Matches. Doesn't matter which order you pick your character just as long as your opponent has the rival character. Both players will unlock the achievement after the match.
Furthermore if you want two or more achievements at once for both players, try the following:
While having Captain America on your team, the other (opposite) player choses Iron Man for their team and you both can get the Whose Side Are You On? (Civil War) achievement as well.If the other player that has M.O.D.O.K and Iron Man on their team, then adds Nathan Spencer they will be able to get Badds to the Bone, the first player (with Captain America) would not get this achievement.The player with Captain America could also add Thor and Iron Man to their team, they will be able to get Avengers Assemble! achievement, but not the second player.I claim no credit for the above achievement links, so please stop by their solutions and give credit to where credit is due.