OK, so I searched all over this forum to figure out if this would work but I just went ahead and did it myself.
You only need one other person to get the Tourney Win achievement. This is how...
Host a tourney with 4 players, 1 min quarters, single game series, and whatever other options. Find one buddy to trade with or just play a game legit. The hardest part is getting people to join your tourney. Once you have 4 people, generate the schedule and make your buddy play you the first game. Have him (or her) let you win the quick 1 min quarter game. Then go into the admin options and select "Tourney Tree" I think it is. Then designate a winner for the other game in the tourney. After this step, I think it makes you wait a day before the tourney advances and you can't play the final game until tomorrow, so do just that. The next day, go back into admin options and do the same as before, designate yourself the winner. And again wait until tomorrow. The next day, go back into the tourney and it will show that you wont the championship and the achievement will pop up.
EDIT: As a matter of fact, I don't think you even need to play the first game legit, unless you need the "play a tourney game" achievement. You can use the designate winner trick on all the games as long as you are the admin.