This Scenario is Easy and Possible Thanks to Pokemon Home (read description for context)
Pokemon Home enables someone to fill their Pokedexes from a fresh save file with minimal effort. The final reward from the Blueberry Pokedex is a free Master Ball, which an underleveled, early game team can utilize for an easy Meloetta catch.
Meloetta, in the meantime, is basically the sole Legendary/Mythical in the Terrarium that doesn't require the player catch Terrapagos before gaining the ability to capture it.
(Note: This scenario is also possible without Pokemon Home if you're willing to trade 240 Pokemon from one game to another. - but who really has the patience for that?)
(Note 2: Meloetta will disobey without the proper amount of badges. Though it is good if you're whipping up a fresh save just for Meloetta)
(Note 3: Thank you for reading. I hope you have a wonderful day.)