One of my training partners has a unique backhand stroke. Very different than others. If I or anyone became his opponent, spinny serve or speed serve he can easily can return it very fast plus any direction he wants. I don’t think my serve is too high. His reflex action is very fast. Is there any serve I can try? Or any suggestion can help me to improve my game?
You should try serving short, with heavy backspin to his forehand side. This will be the hardest serve for him to return with his backhand.
It’s possible that he will still try to return this serve with his backhand, but he will have to take more risk and leave himself out of position. If he does try to return this serve with his backhand, then you should also serve deep and fast to his backhand corner.
It will be incredibly difficult for him to cover both the backhand corner and short to the forehand with his backhand. So hopefully you will force him to use his forehand (for short serves to his forehand), or if he does continue to use his backhand, he may start making some mistakes.
The other serve you could try is no-spin serve. Often players can mis-read these serves. Serve short or half-long with no spin to the middle of the table. If he mis-reads the spin, he will likely hit the ball off the end of the table.
Keep experimenting and trying different spins and placements and eventually you will find something which causes this player some difficulty.