OK. I'm editing the message to give some pointers. If you want to boost this achievement with someone specific take in consideration these four points:

1.- Coordinate using timelines. People that look achievements live in super different timezones. The guy I played with was almost 10 hours away. When I was awake he was sleeping, when I went to bed he was waking up. Put your timezone below and coordinate with people for good results.

2.- Take the initiative. You're the one that wants the achievement, so if you want help watch people in the comments, add them and say hi. If you're shy just keep in mind, the worst case scenario is "No" and no hard feelings ♥ But be pro-active. People in these threads won't add you, you add them.

3.- It might suck to hear, but to boost this achievement can be super easy or incredibly complicated. It's all luck. So find builds for good cars and sharpen your driving skills because random people can join your Speedlist mid-run and ruin your achievement even if you put the settings to Invite Only. It doesn't happen always, but better be prepared for a fight.

4.- Have patience. If something goes wrong just keep trying or schedule another run.

Also, to keep in mind. If you manage to get into a Speedlist alone (The game glitches or the people you play with disconnect DO NOT QUIT. Win this Speedlist alone and the achievement will pop-up anyway. I've seen enough evidence to confirm it.

That being said I got the achievement last week but I'm willing to help people until October 10th, 2020. As I said, add and say hi and we can coordinate something.