Take whatever pikmin you want to level, force feed them nectar until they wilt, then send them on expeditions over and over.

The only pikmin I send on fetch expeditions are the ones between three and four hearts (so they can get decor), the ones just below two hearts (they get a bump tp mushroom strength at two hearts), and purple pikmin.

Also, do most recent fetch expeditions first while walking, since the pikmin will return faster and be ready for another expedition sooner.