Thru the PackG 20

This one is a pain. You need to start a race in 24th (last) place, and win the race. It also needs to be done in a public race, not private. The only way to guarantee you're in last place at the start, is to be the last one into the room during the intermission/voting phase, otherwise it's random. It is possible to boost this, but you need 24 people. Here's how it works;
One person sets up a prerace lobby and invites everyone in. They then start the race. In the next lobby, you'll have a vote for the track and a countdown for 45 seconds before the race starts. The person getting the achievement needs to leave the race during this phase, then rejoin so they're the last person in. Hopefully a random person doesn't join while they're doing this. Now when the race starts, that person should be in last place. Everyone else needs to either let them past to win the race (they can still race behind them as long as they don't win to accrue laps/distance traveled in MP), or cross the finish line and then quit. If they quit before crossing the start line, it won't count them as being in the race, since they don't start a lap time. Those people who quit are returned to the race lobby, and wait there until the race is over. This saves having to do the entire prerace lobby setup again. When the race ends, everyone is returned to the lobby, and the next vote begins. At this point the next person getting the achievement leaves and rejoins, and you rinse and repeat. If everything runs smoothly, you should be able to get 6 people done per hour. My session for this game took almost 6 hours due to occasional disconnects, but we got 26 people the achievement by the end of that time.