Pocketbike Racer

The fastest way to boost this is to set up races at the BK parking lot, 1 lap, 150cc bikes. A lap should take 20 seconds or less. You can do this in ranked or public matches. You do not have to do all 25 at once, just 25 total. And you have to complete 25, not win.

Also, if you're not boosting, good luck finding someone on LIVE.

These other solutions work, but this is much faster. Find a boosting partner and create an Ultimate Cone Trial match at the Parking Lot with 5 points to win. One of you hit the first 5 gates and the game will be over in less than 5 seconds. Then just ready up and start the game again. This can be a public or ranked match. You do not have to win, just play 25 games and you will get the achievement. There will be a counter in your game that says how many have been played.

I found that the fastest way to boost this was to set up a battle royal, 3 hits, in the BK parking lot. Have one player go through all 4 blue gate(cones) and then stop. The second person will drive up in front of the first person. The person who went through the gates should activate their shield, and slowly drive into the other player, pushing them a little bit. It will score a hit, and make the person spin out. When the person stops spinning out, it will score another hit, and then a third time.

If you went through all four blue gates, you should have enough to activate shields again, just in case you didn't get all three hits in time.

Each round took 11-13 seconds for me to complete, and when I finished 25 rounds, the achievement unlocked.

Edit 9/28/11 - Credit to Bodz61: It doesn't matter who wins the rounds, as long as you finish them you get credit.