I know that Cherrim has another form but I don't know how to get it, I am not really bothered about using Cherrim so it's really just for the Pokedex.
Uh... they're both the same Pokedex entry. Cherrim sun form is just found by being in sunny weather, "under the effects of Sunny Day". Hope I helped!
Cherrim only changes to sun form while it is in-battle, and sun is in effect. Otherwise it stays in its regualr form.