HOT WHEELS UNLEASHED

If you have yet to start this game, the absolute best solution is to complete the story mode, and save every single Blind Box and all the coins you get through the career. You can beat every event with the Bad to the Blade car you get about halfway through, and costs 500 gears to upgrade.

If you save every box from career, you'll end up with around 40 boxes. Common cars sell for 300 minimum each, so if you got 40 common vehicles you'd get 12k coins back. This equates to 25 boxes.
If you do this method 2-3 times you'll have 100 boxes in half hour of opening and selling your cars.

This can also be used to obtain 100k coins. Buy all the boxes you can, open boxes, sell the cars back. If you get lucky with legendary and Super Treasure hunt vehicles, this is even faster.

Edit: it's wisest to do this by save scumming your file with the cloud saves. If you get nothing but common vehicles every time you might fall short of the 100k in one save. Everytime you reload your save your progress resets to whatever it was at the time of your save ie, if you opened 1 box before the save, but 100 after the save, you will be back at 1 progress.

Do the usual method of boosting with a friend in private matches on start/finish piece only tracks to get the required amount of coins the quickest. You'll get a bunch of these from the campaign but they cost 500 coins if you want to buy more. You will most likely unlock this achievement way before any of the higher coin or gear achievements, so don't worry about this one too much.

Newer players, read this:

The absolute quickest way to get to 100 boxes for this achievement is to sell all your cars instead of dismantling them so you get coins back instead of gears, but I highly recommend that you resist doing so.

Selling all your cars for coins is terrible for you in the long-run if you're going for the completion, as coins are plentiful if you boost, but gears are not. Only worry about gears.

The most efficient pipeline of resources that includes boxes should go something like this then:
Boosted Coins -> Boxes -> Dismantle extra cars for Gears

You will acquire a significant number of Blind Boxes by playing through the campaign and DLC, all of which are currently included with Gamepass.

Completion of the base game will reward you with 38 Blind boxes. In addition, each DLC will award you an additional 16 Blind Boxes each for a total of 48.

Assuming you complete everything, you will end up with a total of 86 Blind Boxes by the time you have finished, meaning only an additional 14 need to be purchased which should be easily affordable at this point.