Sceptile Leaf Blade
The Battle Tree is obviously a bit different since the game forces you to basically win hundreds of times in a row for the best rewards. I'd say that's more fault of the system of the Battle Tree than the actual game mechanics. I think 200 wins for a Starf Berry is too many, I'd rather reduce that number to say, 50 wins, and to compensate increase the AI competence further as you go higher and give the AI better movesets and pokémon (instead of stupid stuff like Double Team Rotom or Rain Dance Alolan Marowak). Because the truth of the matter is, no matter what team you have or what pokémon you bring, you aren't going to win every battle. Sometimes you do lose to raw luck, whether that is evasion, sleep, confusion, that Scald burn, or whatever, and sometimes you simply get faced with an opponent you're ill-equipped to deal with. Every team has its counters and weaknesses.