I progressively gets harder over time.
#1-7: very weak pokemon like baby evos and such; #7 may hold things like Hitmonchan and such, but movesets aren't great.
#8-14: Difficulty is still easy; decent pokemon with not very good movesets. #14 holds same difficulty pokemon with better moves
#15-21: Decent pokemon with somewhat challenging movesets. #21 is/are ace trainers. This is where competition with big pokemon starts.
#22-28: Ace trainers and the veterans around that area mixed. #28 holds good pokemon with challenging movesets
#29-49: a couple sets mixed in, this is where most people get. Basically just progresses slightly harder over time. #49 MIGHT hold legends. Happened to me.
#50-57: Haven't got beyond this; VERY difficult stage in which legends are mixed with pokemon. Good luck.
That's proof it progresses. I got to 56 on double and 51 on single, so I can't go past my experiences.