Ok there are a few ways to get champion ratings.
First off a Champion rating is given for overall performance in a ranked game, it is not based on points although the person with the most points in the one most likely to get it.
The first way to get one is to play in a team game and be the best performing player in the game. You to this by capturing points, killing enemies and crucially losing as few units as possible.
The second way is to play Head to Head games and win. Assuming your opponent doesn't quit early and you get a good lot of kills and victory points you'll get a champion rating easily.
For those of you who aren't very good at the game or just want the achievement quickly there is a way to get it by losing. What you have to do is pick a fast commander, the Warp Spider Exarch with his teleportation is a good one, and as soon as the game starts send the commander to capture the nearest point. It can be a victory point, power or resource point. As soon as you have it concede the game.
If you manage to do this before the enemy captures a point you'll get the champion rating because you captured the most points and didn't lose any units.
Be warned though you may piss off your opponents (or they may be happy for the victory) and your true skill will plummet. However you will get a champion rating and one more game played with the race of your commander.
For what it's worth I'd say the Head to Head wins method is the best way of doing this, the quitting method is boring and annoys other players and the team game method is slower.