Motorcycle Club

You build up a multiplier by doing consecutive stunts without crashing, and in quick succession. You can keep doing one type of stunt to maintain the multiplier, but you must alternate stunt types to build it up. For example, jump > jump > jump will only be X1, but wheelie > jump > wheelie will be X3.

You will likely get this naturally, but it can be tough because any impact with track walls or other riders will reset your multiplier, and doing stunts quick enough can be hard. However if you need to try for it, the best place is the one I have previously mentioned; in a Kotoma Port race. This track has a nice section near the end with a series of 4 ramps and rings. These ramps alone will get you a X8 multiplier, so all you need to do is a stunt before and a stunt after. Approaching on your custom bike may mean you have a top speed stunt going, otherwise near miss the wall, or do a wheelie. Then successfully navigate all 4 ramps and rings, and near miss the wall or wheelie at the end again. This will net you the X10. If you don't quite do it, maintain the multiplier by doing near misses with the wall and pulling a wheelie all the way up the following hill to the next set of ramps.

I did manage to pop this online so it can be unlocked there too.