Whomever told you that PayDay old works in trainer battles, is dumb is hell! It DOES work in wild battles, and you would have seen it had you not clicked your buttons too fast.
Primarina is definitely the best starter for a play through of Alola, especially in Moon and Ultra Moon. It’s so helpful because it’s got super effective damage for every Kahuna if you teach it Disarming Voice as a Popplio, and it’s also incredible for the first Trial against the Totem Alolan Raticate if you have Disarming Voice on it.