*** Sorry, folks -- servers are down, so this achievement is unobtainable. Original solution follows. ***
Easy to grind this achievement out, as you always earn EXP if you complete the competition (see below). You need 100 EXP to earn Amateur status.
The formula for EXP is roughly as follows:
WIN = 16, 2nd = 12, 3rd = 10, 4th = 8, 5th = 6
Correspondingly, smaller matches will give you more EXP for last place.
However, you MUST complete the competition to earn points. This means, for most trick gametypes, to actually land a trick according to the requirements. If you fail to land any tricks, you will DNF and earn no EXP.
It is very difficult to win trick events online -- they are more popular than the races, and especially in the Spot Battle, players have mastered the endless flip-manual-flip chains.
The only event I would specifically avoid is SKATE, as those matches can continue forever if your opponent is setting tricks and decides to keep doing easily landed tricks. Jam and Best Trick are probably your best chance to win, but Spot Battle events are available all the time for times when you cannot find the other match types.
EDIT: To check your EXP, go into "Xbox LIVE", "Community", "My skate. Profile". At the bottom, it will show "EXP Earned" and break it out by Trick, Race, and Total EXP.