Go All Out: Free To Play

It is pretty hard to find online players so you might need a partner/alt to achieve this one.

For fastest way to achieve this, create a game on "Bag Bounce" mode. Before starting the game, don't forget to set "Punches" to 5.

Once the level starts, instead of the opponent, go for the money bag and hit it 5 times to quickly to end the round.

Repeat until achieving 50 rounds.

Note: If you find it faster to disconnect & rematch, that approach also works.