Update to add: Servers were shut down in early February 2011, making this achievement unattainable from here on out. :( The original solution text follows for purely historical reasons.
To get the Spoiler achievement, you need to win a ranked match against someone five ranks higher than you - which means that, if nothing else, you have to be ranked and your opponent has to be ranked, too. To see if you're ranked: in the game, go to XBOX Live > My NCAA Online > View Player Hub. In the rotating stat up at the top of the screen, you'll eventually see a "RANK: xxxx" value. If your rank is an actual number, you're ranked! If your rank is "-", you're not. (On this screen, you can navigate to the Career Stats page and see how many Rank Points you have; as of this writing, you need about 540 to become ranked.)
The fastest way to gain rank points is to (a) play as a really crappy team, (b) play against a really quality team, and (c) blow them out of the water. (You'll gain points even faster if your opponent has a high rank as well, but that's not as important as the three items listed here.) Depending on your opponents' ranks and how many games you've played before trying to rank up, you may have to do several crushing wins in a row to become ranked, but you'll get there soon enough.
Although you can do Spoiler boosting with as few as two people, it's easier if you have a team of four or so to work with. In that case, the optimal boost strategy looks something like this:
- Booster1 gets crushing wins against Patsy1. Meanwhile, Booster2 gets crushing wins against Patsy2. Continue getting crushing wins until Booster1 and Booster2 are both ranked and more than five ranks apart.
- Have Booster1 and Booster2 play each other in a vanilla full game, with the lower ranked person winning out to get Spoiler.
- Double-check to make sure that both Booster1 and Booster2 are both still ranked and that the loser of the match has a sufficiently lower ranking than the winner. If anything's amiss, have Boosters crush the Patsy players as needed to get your ranks back in order.
- Have Booster1 and Booster2 play each other again, now with the other person winning out to get them the Spoiler as well.
- Now swap places: Patsy1 and Booster1 swap roles, Patsy2 and Booster2 swap roles, and you pair off / rank up / strategically lose in the same fashion detailed above.
This also works just fine if you have three people, with one person playing both Patsy roles. Or if you have the ability to temporary gold-member a couple of your silver accounts, that's also great: Booster1 has Patsy2 as their secondary account, Booster2 has Patsy1 as their secondary account, and play through things as above. It's just more inconvenient and takes longer that way.