This achievement requires you to complete a 3 lap race on the longest course in the game, "The Goliath," using a Forza Edition car, which you can identify by their Pink, Purple, and Blue gradient border (the name should also contain "Forza Edition"). Forza Edition cars are typically obtained through Wheelspins, which you will earn as you play the game. If you have a lot of credits and have somehow not obtained a Forza Edition car yet, you can try looking on the Auction House for deals. There are also several houses in the game that give you Super Wheelspins when bought, which are far more likely to give you a Forza Edition car than regular Wheelspins.
While any Forza Edition car will work, I'd pick the fastest one you have and fully upgrade it so the race goes faster. For reference, I used a 2010 Pagini Zonda R upgraded with Don Joewon Song's tuning package and each lap took right around 9 minutes.
**Although most unlock this achievement without issues, it appears to be buggy for some. My best guess is that it's some sort of intermittent server/connection issue, since people have reported back that they used the exact same car and settings as I did it still didn't unlock (Thanks to Nollie37 and TEN SQUARE 3 for the info). If you've followed the instructions exactly and it didn't unlock for you, I would recommend trying a force a restart of the game and maybe waiting a little before trying again. So far everyone who reported it as not working was eventually able to unlock it.**
Getting The Goliath course
There are two ways to complete a 3-lap Goliath race. The easiest way by far is to use a community created blueprint which you can find in the pause menu under Creative Hub -> Blueprint Events (Big thanks to SWYD1 for pointing this out). Make sure that the race is at least 3 laps.
If you're having trouble finding a blueprint and no one you know can make one for you, you can use one that I've created. The instructions on how to do this are below.
1) Go to Creative Hub->Blueprint Events (The big blue box in the top center)
2) Hit the "View" button (left of the Xbox button) to search, and enter 104585455 under the "Share Code" option. Alternatively you can enter my gamertag "Mitch Connor" under the "Creator" option, but you will need to set the "Route Length" option to "Unlimited" for the race to appear if you do this (thanks to b0seiju for pointing this out). Leave all the other inputs at their default.
3) Select "Reaping The Rewards Short" which is a custom route that is much shorter than the full Goliath course (Thanks to Funky Monkee for confirming this works). It has no checkpoints, so just drive around the starting post.
Note: Playing a Co-op race doesn't seem to work for unlocking this achievement (Thanks Yeast for helping confirm this). The only thing that hasn't been tried yet is playing co-op when all players in the race have unlocked the Goliath themselves, however I doubt this will make a difference so try it at your own risk (if you do try this and can confirm one way or the other, let me know). Doing laps in Rivals also does not count towards this achievement. (Thanks BestAphid68996)
Unlock it yourself
The second way to get the Goliath is by unlocking it for yourself, which is much more time consuming. If you really want to do this, The Goliath unlocks when you reach Level 20 in the Road Racing Series. This will take you far longer than actually racing The Goliath itself. Completing all of the Road Races in the game should get you to around Level 10, and for the rest I recommend repeating long races with upgraded cars and Influence bonus masteries.