EDIT: If you downvote, please consider leaving a comment to let me know what needs to be changed.
This achievement didn't prove to be too difficult to unlock. The starting location is marked on the map as "Eddie's Big Villa" in the north-east of the province.
The finishing point is at the location that is marked on the map as "Arturo's Bar". To find this location, look towards the south-west of the map in the region of a lake called "Laguna Colorada". For anyone who has completed the campaign,
To unlock the achievement, start off at Eddie's Big Villa and reach Arturo's Bar in less than 8 minutes. Some advice for doing the speed run:
1.
Don't use the monster trucks. Other guides have suggested this, but I found that the large trucks are too unstable to handle high speed corners well. They do fairly well on the straights when you can boost and are hefty enough to take damage from crashes and rough terrain, but I found that you lose a lot of time through the corners in the top-heavy vehicles.
2. At the starting location, there should be at least one car that has boost available. These cars can't take as much damage as the monster trucks, but they're more agile and are better in traffic and corners. I found that these cars can typically set a better time for this type of timed challenge than the trucks can.
EDIT: I noticed from a comment that I hadn't mentioned how to start the timer. The timer seems to start automatically as soon as you leave the area around the starting point.3. For my run, I stuck to the route suggested by the in-game GPS. I know that the advantage of the monster trucks is that they can take shortcuts via rougher terrain, but it is too easy to take one of these shortcuts that ends up costed you time because you crash into rocks or can't take a straight line from one road to another. The suggested route will seem longer, but has fewer opportunities for things to go catastrophically wrong during a shortcut.
4. Use boost liberally, but make sure that you're aware of what's ahead! More than once I hit trucks, concrete barriers, rocks, or buildings by boosting too close to an obstacle. That being said, it's easy enough to judge which corners and bends are gentle enough to boost through and which require caution.
5. A general tip for all gaming situations - don't let yourself get too worked up by failures or small crashes. Getting worked up makes you tense, which makes it harder to concentrate on the road ahead.
EDIT: As mentioned in the comments below, don't change cars during your run. If you change cars, the achievement probably won't unlock.
All said and done, this wasn't too much of a challenge. I think I managed to unlock this on my first (or maybe second) run. Good luck!