Scarlet and Violet don't have level scaling, so you should probably just go in the order of level progression: Katy, Brassius, Iono, Kofu, Larry, Ryme, Tulip, and Grusha. Some of these are fairly interchangeable, though -- for Fuecoco specifically, you could probably get away with fighting Grusha before Tulip and even Ryme. Katy and Brassius are each fairly easy to take on first as well.