Not sure if anyone reads this anymore, but I have a good tip that I hadn't seen anywhere for the online championship achievement. Some of the instructions are a little vague as I'm reciting this from memory, but both myself and my buddy got the achievement this way.
- Create a tourney, 4 players, 1 day interval, 1 inning games. Make sure it's NOT private. Call it whatever you want, but putting "boost" or something similar in the title can help.
- Get 3 other people to join the tourney. If you can't find 3 other people, try the achievement trading threads on your favourite forums. Additionally, pop into the online lobbies and if there's anyone there ask them to join. Even as I write this (Nov 3 2008) about a year and a half after this game was released there's still a few people in the lobbies more often than not.
- Once the tourney is full, generate the schedule.
- Under Admin Options (I think, this part I'm reciting from memory), you can actually declare the winner of a match. Do this for the 2 series, picking yourself as the winner in your game, and whoever you want for the other.
- Advance the schedule to the next round (it's X on one of the menus near here)
- Again, pick yourself as the winner of the final match. At this point the tourney is complete and you are the winner.
- Back out of the tourney screen until you're at the main XBL screen, then go back into the "my tournaments" section, pick your tournament, and you'll see a trophy and the message "Congratulations! XXX won the championship!" (where XXX is your name), AND THE ACHIEVEMENT WILL POP.
For those who've played the NHL 2K series of games, you'll recognize this trick as it worked in those games as well.