I originally posted this on X360A but since it was well received I thought I'd add it here too. Sorry for the wall of text but I put this together with the level of information I would have liked when I was going for this achievement.
Rider - Jay Valdes
Engine - Spark Racing 250cc 2 Stroke (D Class)
Stats - 11, 23, 48, 25 & 44
Track - Glamis
Some have said that starting a new World Tour puts the AI into an easier difficulty setting. That's only half true. It's the tutorials at the start of the tour that does this, not actually wiping your save file and starting a new tour (which forces you to redo the tutorial so...). Before starting, take all the upgrade parts off your ATV (as in pick any parts you like but don't choose the +1 or +2 and also don't use the Enduro/Sprint sprocket kits either as they're classed as upgrades). The reason for this is the XBox ramps the difficulty as soon as you put any +1 or +2 parts onto your ATV. Play the full tutorial and then go straight into Stage 1 on the track Glamis. As everyone else has said, there's luck involved as well as your own skill. I came within a hair's breadth 4 times before I actually got it and even had a crash on 2 of those attempts. I raced properly for the first 2 laps and then stopped on the final bend at the end of the 2nd lap. Now grind your teeth and complain about how long it takes them all to catch up. Start moving (I was actually in 16th place when I completed the 2nd lap), round the first bend to the right and under the bridge. As you approach the next jump, angle to the left by about 40 degrees and you'll be on a shortcut. Granted this isn't a big shortcut but on the race when I got the achievement it made the difference. I wouldn't have got it without that shortcut.
Credit goes to CheshireMulisha for telling me to do this on Glamis.