Consider all of your options for a strategy when you are choosing your team. If you think an opponent has a good strategy, change your team to theirs when you beat them. If you are playing doubles then pick your first two mons as things that can compliment each other. If you’re playing singles, you might be better off using a varied team.
Balance your team. You need lots of different roles on your team. For example, you don’t need to have multiple stallers or sweepers. I would recommend something that can heal reliably.
Be prepared for the frontier brain, Thorton. He generally uses some of the best mons with the best strategy. If you don’t want to battle him, do something else between each round of matches (there are seven matches to a round). You will always battle Thorton on the last match of the third consecutive round.
Some of the best mons:
Sceptile - great sweeper, has priority and potential to screw the enemy up with screech.
Medicham - although you may not think so, this thing is amazing when it’s used right. You just have to use endure, then reversal and keep spamming reversal.
The pseudo legendaries except for Dragonite - great potential. Especially Garchomp which can activate its sand veil ability with sandstorm.
Milotic - especially with attract. I think it might have recover as well which makes it even better. It can both stall and sweep to your heart’s content. This is probably the best and safest mon on this list strategy-wise.
Eeveelutions - all of them are great. The best ones would be Espeon and Leafeon though as they can both set up using calm mind or swords dance and then use baton pass to send the stat boosts to a different member of your team.
NOTE: I’m not including the lvl 50 mode because the it doesn’t really matter what you do in them (they’re much easier!)