You need dashell to do this
1.go get 10 shock shell at flipside store on 2f
2.go to world 8-4
3.use mario flip right away go to the third ?box hit don't get the slo-mo go to the end by the door flip back to normal then get it.
4. As soon as you get it use a shell shock facing the way you came from mess up on the pointing wii remote to get the big shell not helpful you want smallest shell possible
5.follow the shell don't let it out of your sight all the way to the beginning this will give you 100,000-130,000 experience
6.an rinse and repeat as much as wanted
Hope I helped