Forza Horizon 3

The method I used to grind out the Hot Wheels skills was to use jumps to get enough air to get a Hot Wheels Air skill. A Hot Wheels Air skill is one step above Ultimate Air so basically get as much air as you can without crashing and it should register.

I went to the Rusty Reef area of the map and found a series of sand dunes I could repeatedly jump over to make progress. I had about 70% progress and it took me a bit over an hour to get the other 30%.

Here's a video showing the technique I used:


(I've updated the clip from the original version using the more optimal location suggested by Blue n Gold Eel)

Note that you will want to err on the side of caution while initially figuring out the best speed to take the jumps at as I found that when I crashed I respawned on the track and it was a pain to get back to where the dunes were. Once I figured it out and got in a rhythm I didn't have any issues with crashing.

As pointed out by Phil the Bear you will also be getting kangaroo skills while jumping the dunes so you will make progress towards this achievement as well:

Larvi's solution is great and is especially good if you still need Kangaroo skills as well. However, I've found a faster way to grind out just the Hot Wheels skills.

As many have pointed out, most of the Hot Wheels skills are the next step above an Ultimate skill. I've found that skills unique to the Hot Wheels DLC also count toward this achievement, including Speed Boost and Loop-the-Loop.

Update: I've found an even faster way than my original method. With this method, you'll get slightly over 1% (21 skills) progress for every 7 iterations. This works out to 1.05% progress in under 2 minutes.

I've left the original method below as an alternative, but here's the faster option:

1. Travel to the Half Pipe Climb circuit race, which is an S1 race in the Thrill City area (lower left of the map). This race is just southwest of the Beach Blast Danger Sign.
2. Press to set up the event. Choose Hot Wheels Campaign, then create a blueprint.
3. Choose Stunt Swap, change the stunt to a Boost Jump, and then choose Create Blueprint.
4. Start the race. To make this as easy as possible, you'll want to get ahead of the other cars before the end of the halfpipe.
5. After the halfpipe, you'll go through a left turn. Immediately after that turn, hit the speed boost, the jump, and the speed boost right after the jump. This will give you three skills.
6. Once you see the second Speed Boost skill pop up, press to rewind. Rewind three times, which will put you a little in front of the first speed boost.
7. Resume. Go through the two boosts and jump again, giving you three more skills.
8. Repeat steps 6-7 until you get the achievement.

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Original loop-the-loop method
Using a loop-the-loop with a boost in front of it will get you two skills each time. By using rewind, you can gain 1% progress for every 10 loops. I timed this and found that it takes me about 2.5 minutes for each 1% progress. With no waiting for landing jumps and time to turn around, this ends up being faster than the Hot Wheels Air approach.

Here's how I've been making quick progress toward this achievement:
1. Drive or fast travel to the Skyscraper Heights Drift Zone, which is located to the right of the loop in Thrill City.
2. Drive through the drift zone, toward the loop. You'll hit the speed boost and fly through the loop.
3. Once you see the Loop-the-Loop skill pop up (Awesome or Ultimate, it doesn't matter - both count), press to rewind. You'll want to rewind three times, which will put you just in front of the speed boost.
4. Resume. You'll hit the boost and go through the loop again, giving you two more skills.
5. Repeat steps 3-4 until you get your achievement.

Here's a video showing the location I used:


I used 1979 Hoonigan ‘Loki’ K5 Blazer with all upgrades.

You can use any car, but it is best to use one that is off road and has a good acceleration.

There is a much, much faster method of getting this than any of the others mentioned here, and I'm surprised no one has mentioned it. Thanks to nightw0lf for noting that the Speed Boost and Loop-the-Loop skills from the Hot Wheels DLC also count towards this, I did some simple experimenting. The entire achievement can be earned in 15-20 minutes total using this method if you were starting at 0% progress. I earned my last 60% in about 10 minutes.

Summary:
Just keep driving back and forth on top of a Speed Boost platform without driving off of it. If you don't drive off of it, it doesn't launch you away, but still counts each skill (2 skills per movement actually if done right).

More detail:
Simply drive slowly onto a speed boost platform and stop on it. Drive forward while on the platform and stop before driving over the edge of the platform so that it doesn't actually boost you away. You'll hear the sound of it trying to boost you. Each time you hear that sound, it gives a Speed Boost skill, which counts towards this achievement. Move in reverse back across the platform again and stop before backing off the other end of the platform. You'll hear the same boosting sounds in that direction as well and get more Speed Boosts. When done the most efficiently, for some reason it gives 2 Speed Boost skills moving forward, and 2 skills moving in reverse, unless you don't move very much, then it only gives 1. Regardless, they add up very quick using this method once you get the hang of driving back and forth quickly, and you can also rack up an extremely high skill score using this too. Also, if you don't actually stop after each movement, the platform tends to boost you off of it.

For those who don't own the extra Dlc packs, which would also mean a fair few new achievements to complete to get 100% for the game, the quickest way is to do a 1 lap race on a short circuit.

The 2 quickest I've found are a cross country circuit race at the Timber Mill and also at the Gold Mine. Both these tracks are about 1 mile long. Go into the Exhibition section if my race doesn't show, create a 1 lap race. For the Gold mine use Hyper Cars and Offroad for the Timber Mill. They take about 45-55 seconds to complete.
Also load the skill perk that lets you start the race in 1st place to make it quicker.
Still going to take a while, I have about 550 races to do! 😜