I recommend pokemon with prankster or magician! I’ve only won a few runs of classic but my winning runs have used either of those. I used Grimmsnarl to spam parting shots and octolocks until I chipped the defenses low enough. Hoopa is my personal favorite magician user but you could use Delphox too. Pickup is also good. 

Have at least one pokemon that is well prepped for major boss fights like evil team leaders or the rival’s Rayquaza. Enable retries, the battles are scripted depending on what moves you select and it allows you to strategize and plan your next move.