This solution will be looking at the boosting side of things.
Also if needed your also unlock the achievement for simply playing a PVP match the Welcome to the World of PvP achievement.
Can be done with only two people, but more might make this go faster. If any one does this with more people let me know the results and if it works I'll give credit and add it into the solution.
This method does work but will take around 2 hours for both people to unlock the achievement.
Each round is 10 minutes long, so unless your over powered your not likely to deal the 3,000,000 damage to win the round.
At the time of writing this solution, the current MP is dealing damage to the target in the middle of the arena.
So what your want to do is head to the Governors office/lobby and go to the left side kiosk and speak to the NPC behind it. What I selected was...
Avatar Battle (Uses your avatar)
Set the lobby to private and launch, after loading your be taken to the waiting area. Take a moment to invite your boosting partner and while they are connecting head over and select any of the missions
NOTE: When inviting your boosting partner just invite them to game and to not form a team. If you click form a team don't worry you get move them from Alpha to Bravo by going into team information.
I'd recommend equipping either a Sniper Rifle or Assault rifle with range so it saves you getting face to face with the boss. Once your both in the lobby, ready up by pressing
Once your loaded into the match just have the person going for the win shoot at the target in the middle, the team that causes the most damage will win the round. Once the match finishes your be taken back to lobby where you can ready and continue. Take turns or simply let one side win all 5 games and then switch. As mentioned above the game does last 10 minutes so might be better to switch between rounds to save boredom. Then simply rinse and repeat for both people and then you should both have the achievement in around 2 hours.