Roundabout

Although you will likely get this normally through the campaign, I found an effortless way of earning this achievement. In fact, you can even get it as your first achievement since it can be done in the Ball Bouncers challenge which is already unlocked at the beginning of the game.

From the main menu, go to "Challenges" and press . The second one called "Ball Bouncers" will already be unlocked so select that challenge and press again. Either watch or skip the cutscene with and the challenge will begin.

Instead of actually trying to bounce the ball (which is the objective of the challenge), purposely move away from the ball in any direction and let the ball fall to the ground. You will start to notice your multiplier increase and then it will say "Challenge Complete". Press to play the challenge again. You will notice your multiplier is still at the same number it left off at when you "completed" the challenge! Basically, just rinse and repeat this method until you reach a 35x multiplier at which point the achievement pops immediately. Remember, don't actually try and bounce the ball on the car, let it fall to the ground right away, restart challenge, and repeat... simple!

A good place to do this during the campaign is in the "Pick up Disco Sylvain" mission in the city. This is the mission where you have to take him to the Memorial and jump across a bunch of spinning barges in the water to get there.

Since the water is flat, there aren't really any obstacles to bump into, which makes it easier to collect the coins as you jump from barge to barge. When you get to the dock on the memorial, you can turn back around and go the other way, since the coins will have respawned. Go back to the beginning and repeat this as much as you need to in order to get your multiplier up. You'll probably need to do some boost jumping (hold a trigger while you jump) to get between barges fast enough to keep your multiplier from timing out.

Once you get to 35x, keep going since you need to get it up to 48x in order to unlock a powerup and get this achievement:

Here's a video guide on how to easily get a 48x Multiplier streak which unlocks the Multiplier Freezer upgrade and the 'Cold As Ice' achievement + You'll also unlock the '25 or 6 to 4' 35x Multiplier achievement along the way!
This also is a way to farm money making it quicker to earn the 'Possession Obsession' & 'True Maximilian' achievements.
Bonus tip explains the 'It's Tuesday' achievement at the Memorial
This isn't a plain guide, it's with the @FromTheHipMen spin!
Hope it helps someone out! Cheers