After watching a lot of videos they all say:
Give 2 to 3 rainbow beans and Level Up, but I did that and my Cleffa won't transform
Does anyone else have more tips and tricks?
Pokebeans raises the affection of the Pokemon, not the happiness. The fastest way to raise happiness is to get Evs reducing berries to lower EVs but raise happiness they can be found on route 10 on ula'ula island.
get the EV reducing berry that your Pokemon doesn't have any Evs in.
Pomeg Berry – Lowers a Pokémon's HP by 10 EVs.
Kelpsy Berry – Lowers a Pokémon's Attack by 10 EVs.
Qualot Berry – Lowers a Pokémon's Defense by 10 EVs.
Hondew Berry – Lowers a Pokémon's Special Attack by 10 EVs.
Grepa Berry – Lowers a Pokémon's Special Defense by 10 EVs.
Tamato Berry – Lowers a Pokémon's Speed by 10 EVs.
U can then put the berries in poke pelago to get more of the berries
Source: I did this myself