Deformers

If you have a boosting partner, or a 2nd Xbox (not X) with each having a 2nd controller, you can do this with no problem. Custom matches do not count any longer towards your total, so it has to be 2 different accounts and it has to be at least 4 players to start a Formball game, hence the 2nd guest controller for each player.
Load up the game and have both players select Formball and start the game. Take turns going from the middle and scoring. Once player one scores, player two should be in the middle ready for the ball and run it in for the goal. You should be able to get 8 goals each if you do it right per match. Make sure the main account and not the guest is the one making the goal.
So here is the hard part: if your 2nd controller is idle for 1 minute, it kicks you from the game. Putting a rubber band on the controller to move DOES NOT WORK! You have to have make the guest move at least every minute and it has to be in different directions, the developers made sure you couldn't boost the achievements easily. What I did was alternate to which controller went back to the center to get the ball, but made sure my actual account was the last to touch the ball to score.
Good luck, give me a positive vote if it helped.