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.
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.
-------------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.