I made this quick guide to help people that have trouble completing the challenge mode, since I had the same problem and it was really frustrating. I kept playing and I felt like the cpu was just programmed to always beat me but I found a solution that helped me to complete the challenge mode easily, so I'm going to share the solution and make life easier for people that can't complete challenge mode without this trick!

I found this trick that can win you all the normal (1v1) matches vs a cpu character. All you have to do is manage to win the first point by yourself or just manage to be ahead for at least 1 point in the match and it will be a secure victory. If you manage to be 1 point ahead, then every time you are serving the ball, wait for it to come down, and then hold the right button to move to the right, while attacking the ball, after 2 attacks while running towards the cpu, the cpu won't react fast enough to hit the ball again, and thus, you get a secured point, just like shown in the video below.

At some point in the challenge mode you will come across a cpu that looks like you but in a dark gray colour, that copies your movements. Many people find it hard to win against this cpu, but if you use this trick, it will be even easier than vs normal cpu characters because each time the cpu is serving, you can go to the right side and he will move to the left, so you can start to serve all the time, without having to lose the ball, as shown in the video below.

I haven't found any tricks that are guaranteed to work for the team level and points mode level, or the final boss, but they should't be hard to go through as long as you are not very unlucky, and if you have a hard time winning those, since you can easily win all the other matches fast, you can reach the level you have trouble with in no time.
Thanks for reading my guide, this is my first guide and I hope it helped, feel free to comment if the trick helped you out and if you have any extra tips or ways to win the other matches, you can ask me to add them to the guide (and mention your name in the guide of course). Moreover, if you found my guide helpful, please rate up so that more people notice this guide!