Besides Cheren, Morimoto, Cynthia, the trainers on the cruise ship, and the trainers at Nimbasa Stadium, where can I go on a daily basis to battle trainer with high level Pokemon?
Those are all the high level trainers in Black/White 1. You can also battle the girl outside the Ferris Wheel in Nimbasa City but I don't think she is high level. The only other thing you can do is battle Pokemon in Giant Chasm.
If you are playing BW2 you can also battle Cilan/Chili/Cress in Striaton City and Colress in the Plasma Frigate.