I would say to utilize the training mode with 1/4th speed to practice the boomerang b-reverses to get comfortable with them. This is to ensure that you perform them consistently, but once you start getting the hang of it, then increase the speed from 1/4th. I can't really suggest any button mappings since I use the default controls except tilts for cstick, but I would suggest to experiment with the controls until you find one to your liking.
This is only for boomerang B-reverses, but wavebounces are much harder to perform with the default controls and would require custom controls to be made easier to perform. You don't necessarily have to have custom controls to do them since you can do them with the default controls, although it's harder to do.
