you can fight weaker players by losing the first 10 games on your account.
Oddly enough this will not mess up your stats because you will get a far better win ratio after that.
Winning most of those will sort of set the level at who you play. If you win all of them, you're going to likely see people whom are on the ladder (rank 1000+, possibly even top 200!)
Losing all of them puts you against some ultra new players.
Here is an example of a game where i troll a new player:
http://www.youtube.com/watch?v=wrvb2DjV1Yg&list=PL595B4752EA471E8C&index=1First 3 vid's cover it. The other game is somewhat cool also.
As the others say, the level the opponent is doesn't matter. How bnet percepts their ability does. The first 10 games seems to be the most crucial stage, as it will immediately put you were it thinks you belong.