Easiest way is to use a 2nd controller, don't even need any CPU players. Play on Plunder Isle with starting point on outside. Play 2min game and costume ball should appear at 1min on the outside of treasure chest ring. No need to bomb anything or worry about the CPU player getting ball. Kill the 2nd controller player and repeat. If costume ball stops showing up, do as xGodDogx suggests and return to main menu and start new match.
Suggestion from Ceroc:
I set it for 9 round, killed player 2 8 times then suicided 8 times then just kept ending the final round with a draw until the achevement popped! Never needed to back out to the menu, played Plunder Isle on Outside.