I'm not sure if there is a way of boosting this, but this is the non-boost way how I got it this night.
Update from erod550:
Boosting seems impossible because you can only do Showdowns against other players. If you are offline the Showdown option will not be available. Also if you go to a Car Meet while in a private match it throws you in with the general public so as far as I can tell you cannot just do a Showdown against your friends to boost unless you get lucky on the matchmaking.
Intro / Prerequisites:
This is an online achievement. There's no option to start a showdown in single player. You can go for this achievement once you have access to all six car meets (which means having progressed far enough in the single player I guess).
How I did it / General strategy:
I'm not sure what triggers it, but after some time waiting at the car meet you can issue a challenge for a showdown via X button, or press X to join if someone else initiated it. If at least two players joined, a point to point race will start after the countdown.
Cars of any class can join, but lower classes get a head start of the higher classes. So for 5 out of the races, I used a A (800) 2011 Subaru WRX STI with Weight Reduction, Tires, Turbo and some other performance upgrades, so I focused mainly on handling.
Having a head start of the S1 or higher cars and a car that handles well (so no bigger crashes) the only competition I had was the other A and B class cars. The faster cars mainly could not use their high speed because of the track layouts. For one race (Nice) it took some tries, for the others it was quite easy. I would consider myself averagely skilled, so it's possible to win one race if you keep at it.
Saint Martin drag:
For the Saint-Martin race (a drag race) you need a fast (If possible X (999) or high S2) car with fast acceleration and top speed, and with my Henessy Venom GT it was possible after a few tries when I got a slower opponent. Your best bet would be a Maserati Tipo 61 (Barn Find), at least they have won every Saint-Martin race when one of those was involved (either fully upgraded or max S1 to try head start).
theMAYFLOWA pointed out that manual shifting helps to gain the few seconds you might need to win, so give it a try if you are losing to faster opponents.
- You can leave, wait 1-2 miinutes and rejoin the car meet to get some other opponents. This is interesting if either nobody joins the showdown or if someone you can't beat joins again and again.
- After winning one you can fast travel to the next one.
Quitters and disconnects:
- I thought winning against quitters counts as well. There was one race where an opponent left mid race and another where the opponent left when the race loaded and I ended up alone on the grid. I finished both and proceeded and the achievement popped. (I'm not 100% sure about this because I had one win before trying this but I think that was at a meet where I did not encounter a quitter.)
- Eurydace reported he finished against a quitter and did not receive credit.
- Lavindathar reported he got credit against a quitter.
- Getting disconnected after winning while waiting for the race to end does not count towards the achievement (thx CTM AudiRS6 for pointing this out)
It might be best to follow your progress percentage on the XBox One achievement app and check after these cases. Please report in the comments what you find out!
Hope this helps, if you have questions feel free to ask.