To be honest, I find the level 9 CPUs harder than most people on Wi-fi, especially CPUs such as Luigi, Peach and Pikachu.
CPUs are more predictable, but they seem to know exactly what moves you're using, whilst a human wouldn't react quick enough, if that makes sense.
With humans, you end up in mind games, and all that sort of stuff. But, with CPUs, you sometimes do an attack, and the CPU miraculously shields in the next frame, or something insane like that. Humans can't usually do this.
If you are finding the CPUs too easy though, there's always handicaps.
See if you can beat L.9 CPUs when you begin at 50%, for example.