Forza Horizon 4

Note in 2023: This guide was written when Eliminator games were full (72 players or close to it.) Things may work out differently in smaller games these days. But I hope the tips are still useful.

I've also heard that there's an increase in cheaters since the game launched on Steam. If there's weird stuff happening, that's probably why. It probably makes winning even harder, unfortunately.

Original Guide:
There's no guaranteed way to get this achievement. But here's a detailed guide to help you out, including some insider tips from the developers. (Brief summary below.)

I strongly suggest playing the game a couple of times before reading the guide, otherwise it's probably just confusing.

Getting to the final showdown:
This is fairly easy. Stay near the outside of the arena and let other players battle in the middle of the map. Drive around and grab car upgrades when you can. Off-roaders are usually your best bet.

Watch your mini-map. If someone comes towards you, try to avoid them. Head-to-heads usually finish towards the center of the arena from where you are! Be ready to react if you do get challenged and start racing immediately - don't wait until the destination pops up. If you face the right direction you'll almost always get a nice head-start. (There are exceptions. But this is your best bet - especially in slower cars!) With this strategy I make it to the final showdown about 70%. (Depends a bit on your luck with car drops and challenges.)

PS: Stopping the car will supposedly "hide" you from other players' maps. I don't think this works reliably and people seem to find me no matter what. You're better off looking for a good car.

Final Showdown:
This starts when there are 12 players left in a reasonably full game (max. 72 players.) Or about 6 left in 36 player games.

With about 20 players left (10 in smaller games,) check the map to see where the final ring will be. The finish is usually towards the center of the map from where the ring is! (There are some rare exceptions. If the final ring is near Edinburgh, the finish often seems to be in the city, for example.)

Once the final ring is indicated on the map, make your way to a road leading towards the center of the map, still avoiding other players. (If the final ring is exactly in the middle of the map itself, you'll have to guess a direction. Go where no one else is and hope for a bit of luck!)

When the countdown starts, get close to the edge of the arena, facing your expected finish. You want to accelerate and time it so that you break through the barrier about 3-4 seconds before the countdown ends! (You can survive outside the ring for 5 seconds.) When the final destination pops up, adjust your course and go!

PS: If you're challenged in the final ring, remember that head-to-heads are cancelled when the countdown starts! It's a valid strategy to ignore the challenge and hope that someone else gets eliminated before you, since a head-to-head at this point leaves you out of position with little chance of winning anyway.

Getting a win:
If you picked the right direction you'll have a good chance. Keep in mind that supercars don't handle well off-road. Stick to the streets if you can. In off-road cars, anything goes.

Check the map occasionally on a straight road or field (take your finger off RT to stop zooming in) and keep an eye on the best route, avoiding lakes, etc. Try not to hit any obstacles (slow down if necessary) and beware of walls around towns! Concentrate on yourself, not what others are doing, and you'll get that win eventually.

Obviously other people can also guess the right direction. But I've been in final showdowns where no one else was near me. They all tend to cluster together. With a little luck they're all on the other side of the arena and you get a massive head-start!

Alternate strategy:
See where everyone else is lining up and go in the opposite direction, hoping you get lucky. :-)

Summary
- Stay near the outside of the arena and pick up better cars
- Avoid head-to-head challenges
- Expect races to finish towards the middle of the map from where you are
- You can leave the final arena 5 seconds before the countdown ends

Extra notes and tips:
Don't play Eliminator when you're frustrated. It's battle royale. Everyone wants to win and no one knows (or cares) about your achievement hunting. Get in the right frame of mind before starting. A full game takes about 20 minutes.

This strategy is NOT for the "Heads Up" achievement (winning without picking up car drops.) But you could certainly use a similar strategy, sticking with the starting car, or winning a head-to-head early on.

Finally, please don't downvote just because you didn't win first time. There's no victory guarantee in this game mode. This is simply a strategy which worked really well for me.

So I won this achievement together with Ransacked - Collect 5 Car Drops within a single game of The Eliminator, in a level 3 Volvo V60 Polestar, and I placed 1/49. The trick is that when you are preparing for the final race (you cannot challenge other players anymore), you may want to bet for a 'right' direction, aka the finish point. At this time it's fine to drive out of the zone a little bit. If you bet right, you will have a perfect head start for this final stretch(this strategy is also suitable for the HEADS UP achievement, which I didn't get yet). The finish point is usually located at the middle of the map.
This 2nd time I won I was in a Audi RS6 '03. As I said, try to guess for a direction and gun it when there's like 20 secs left. However, you may either win easily or come dead last...

edit: since a hot fix is out now, this approach may not work anymore. After the final zone has been settled, drive out of it can be very dangerous cause the bar will go up really fast. A new possible way is that guess the final destination(as usual) and wait at the very edge of the zone. When the final countdown ended, just gun it.