There are several ways to play:
Easy: Use 1 Mario to collect all the coins before setting the others into motion.
Speed: Start the other Marios as soon as the gate is opened. Luck is a factor in my 11630 high score.
DropOnly: Once you put down a pink box, you can't pick it up.
NoTouch: Put all the pink boxes in the beginning, and never touch them again during play. Extremely difficult with lots of luck involved, but it's possible to finish with a gold star.
1Box: Play the level using only 1 Pink Box.
My normal way to play is to set the key Mario to get the card, and as I waited for it to return, I set the other Marios into motion. Wait until they're united, then let them exit. Easy! If you're into speed, though, not so easy. I haven't been able to repeat my highscore. :(