Just based on experience, the best way to be successful at the battle tree is by losing a lot. The thing is, you never know what moves the ai has or which random is coming next so what you do is to just try to reach as high as you can, lose and analyze the Pokemon and movesets of the ones you encountered, as well as their battle style and try to guess the ev spread. Let's take Wally for example. If he leads of with Garchomp, the most probable situation would be he will mega evolve and use sandstorm. That's one of his battle style so if he leads off with Garchomp, I can plan ahead on what to do. During this turn, I can use it to either set up stealth rocks or any boosting move then just kill his entire team. Basically, it's just trial and error until you find the team that is best suited to handle as many Pokemon of the ai as possible. However, I'm not particularly sure if you can prepare for all of them with just one team
I used this method myself and managed to reach forty one opponents in super single but I just rage quitted because my leech seed missed three times in a row. Also, try to switch up your team otherwise the ai is gonna hard counter your team if you stick with it for like thirty battles
