While on the stage you will have to fight both brothers at the same time. You also must avoid 8-bit Shellcreepers that will walk around the stage.
The Shellcreepers can be flipped over and used as a throwing item. If knocked over and left for too long, the Shellcreeper will flip over and turn another color (Green→Purple→Red) and become faster and more damaging (with knockback increase). Sidesteppers will also walk around the stage and try to harm you in the middle of the fight. Always avoid a two-on-one fight. Use the stage to position Mario and Luigi at a point where you can perform a short one-on-one tussle to increase damage. Use Shellcreepers and Sidesteppers to your advantage.
Clearing this event unlocks the stage Mario Bros.