Your oponents on the super battle tree usually use pokémons evolved, fully EV trained and with the right nature for the especialization they are taking, you have to do the same to keep up. In the super battle tree, they often have especialized sets instead of balanced, especially in the double formats. But in resume you have to have your team right buillded in combination with advance battle skills.
You have to be creative, make use of synergy and combos in the righ situations too, instead of just attack sometimes. For example the move Sky attack, it takes two turns to attack and you may be nocked down before you charge it, like solar bean. But that doesn't make it bad, actually if you just give your pokémon a power herb and send him to enter in the end with the function of Finishing moped foes, he will be the deadly killing blow of the team. Giving the right situation, almost anything is important, and the NPC in super battle tree does this alot. If you pain attention on how they do things you can try mimic some of the strategies they are using, is very good when you are still learning. Besides you can also predict which strategy they are using and how they are doing it to disrupt, but trying direct hits in this difficul using standarth movesets, will be like playing gamble, in the super battle tree you have to be smart.
Also another important thing is that, you have to choose the right moves for the job you are doing. For example, if you want moves to kill fast. Since their pokémon have the right natures and EV they often strike first or endure your hit in case the move you are using are not so powerfull. For example using Fire blast over flametrower is important in this kind of difficult because, most enemies won't give you the chance to hit them twice, so killing them in one hit ensure you most survival than trying to hit them twice. A move with 70 base accuracy for example, will hit in 7 of 10 times. If you misses one hit, it will still be better than hit him with a move that won't kill him and give him the chance to retaliate.
But you have to pay attention at everything many of them will try using crowd control in the double battles, or just try to reduce your accuracy, raise their evaziveness. So there are exceptions for everything, in cases like this, moves like magical leaf and aerial ace who haves little power but aways hit is a good backup giving the right situation, in this kind of difficult, you see purpose in almost every move and how use them properlly and in the right situation can be the key to victory.
If you create your first pokémon with a move like fire blast, and bring his cover partner with a move that aways hit can give him a good backup against the uknow for example. But in most cases just following the role your pokémon are made to do work with no much difficult as long as you have the right tools (moves) for the job.
Every role needs the right move, but the way you can employ each can vary alot. Try experimenting with different moves and pokémons, choose the ones that can do what you want/need to do. Almost any pokémon and move can be fine tuned to fill in a particular situation, but some work better than the others with the right combination of pokémons, other moves, abilities, etc. If your setup is not working well, try to rethink what you can improove, and what kind of enemies and strategy you are facing most, what you can do to conter that and more.
If you have interest, try reading some of thoose official guides, i'm sure it will help but the downside is that they have to be brought. But in resume is what I said.
Hope I helped, I wish you luck on the super batte tree.