I just now kicked up Souta at first try. My tactic was to start with Steelix to tank and kill his Skarmory, then I switched it for Alolan Raichu, used Electric Terrain and OHKO everything except Dragonite on which I had to use one ultra potion. It was the easiest battle for me so far. Electric terrain + Thunderbolt did this match for me. Even Drragonite couldn't really harm my Raichu with its moveset.