There is an easy way to get rid of your rivals in Rock Garden and other playfields. Hold B and then press A to pick up a bomb. Then bank one off someones head, and while they are still dizzy pick 'em up and throw 'em off!
Also, if you die and become a ghost, if you ran into another player you will hang on them and you will be able to control them for a while. Use this advantage to make the players run into bombs and (hopefully!) cause a draw.