I've always imagined that in the world of pokemon, there's several illegal underground organisations responsible for the running of Human vs Pokemon fighting matches.

There's a variety of match styles, some where humans are granted the usage of technology, or gadgets to help level the playing field.
Then of course, there's the matches where it's just Man versus pokemon.
Simply unaided in a fight to the death. Somewhat akin to the episode of Torchwood; Combat.
I'd imagine the majority of Pokemon that would be fought would be fighting type or other humanoid shaped and sized ones. But others are not ruled out, with things such as Dragon types being brought in as a sort of final challenge.

I'd like to know your thoughts on this sort of thing in the Pokemon world.
Or just your thoughts on Humans Vs Pokemon in general.
What kind of Pokemon would be a challenging fight?
What the toughest that you would you go up against?