HOT WHEELS UNLEASHED 2 - Turbocharged

Garage > Shop.
This is the in-game Shop, buying any 25 vehicles using coins earned from playing, including "Already owned" duplicates.
Do not buy cheap 2000c duplicates if you're going to go for the achi for owning half the base game's 'Collection' vehicles (100 needed for '...Is half done'🏆: Solution for …Is half done In Hot Wheels Unleashed 2 - Turbocharged), apart from buying 1 duplicate for 'Better too much'🏆: Solution for Better too much In Hot Wheels Unleashed 2 - Turbocharged.
The cheapest way is to buy all the 2000 coin (cheapest) vehicles, then pressing to Update List for 350 coins. The achievement tracker will update as soon as you press to Buy them.
If you haven't bought many vehicles previously you'll need over 50,000 coins - however, you can press on your cars to sell them to return half of their original coin cost - but keep at least your best vehicle of the 6 types (Rocket, Off-Road, Balanced, Drifter, Swift, Heavy Duty) for forced type events and a podium finish with the 6 types for 'Shapeshifter'🏆(Solution for Shapeshifter In Hot Wheels Unleashed 2 - Turbocharged). Tip: Favourite the vehicles (and unsellable Gift Cars) you want to keep, and then you can easily spot the new vehicles that you'll want to sell.
WARNING: If you sell vehicles you'll have no way to be sure if you owned them previously, which is a nightmare if you're going for '...Is half done'!
You'll probably earn coins fastest from the Hot Wheels Creature Rampage 'career' mode, and from Hot Wheels Spins (also in the garage), so use any spins you've won (you can win spins from levelling up, etc).
I did this at L19, only defeating the first 2 career mode creature bosses, and had 6000 coins left (but hadn't sold any vehicle at that point). It'll probably take a couple of days to get 50k in coins (if you wan't to do this in one go, without selling vehicles), or maybe a day if you just play career mode (I play on Easy which gives easy wins, does not reduce prize money/XP, and you very rarely fail to win and move on).