Brilliant Diamond Adventures #10 (Patreon)

In the game, Calista the Shellos was a trade and she grew incredibly fast with all exp share benefits. In fact I often had to keep her out of battle so she wouldn't get too far ahead of the rest. I also didn't want to evolve her until she learned Earth Power which meant a lot of the time I was mashing B when she started evolving. It was worth it in the end, especially as Gastrodon is such a great Pokémon to have.