Just some tips for the ones that are actually challenging. Mainly putting this up for the Wingsuit cheese but will throw some other helpful tips in anyways.Wingsuit
Once you unlock the air brake start practicing with it, it can be extremely useful to be able to slow down on certain parts of a course where the game would otherwise have you bombing through it like a mad man. (just don't take the second upgrade for Wingsuit brake while you're doing the challenges
The next tip is what allowed me to get 5 gears on every wingsuit challenge. On certain courses you can abuse the low flying bonus. You do this by finding a relatively level part of the course and just simply find 2 ideally elevated reel in points to keep yourself moving and fly in circles until you're confident you have the points to finish the course with the points you need. Every wingsuit challenge i could i did without using this, but some of them are just insane.Racing
Check each race before you start, if it lets you use sports cars use the Mugello Farino Duo. I was using this even on the offroad'ish tracks and it just makes things so much easier.
Don't get too stressed about the driving challenges early on, driving challenges are supposed to be doable with what you have when you find them using the default car but if you are really struggling just return to it later when you have Nitrous II.Flying Races
Come back to all of them with the CS7 Thunderhawk for planes and the CS Navajo for the helicopter ones. Just remember with the CS7 Thunderhawk, if you hold down the acceleration your nitrous will never recharge.
Again like with the driving races, getting to the point where you have Nitrous II for planes & helicopters will make things easier.Sea Races
Destroying vehicles around the base will keep your multiplier up. One trick i used was to group up all of the vehicles i could and destroy those first to get my multiplier up before i destroyed the big earners.
Check the base when you start for choppers, if you find one, use it. Surprisingly a lot of them do which kinda makes the challenge pointless.
If there are no choppers, there will be vehicles with turrets around that you can position in places for best line of sight of the chaos objects. For challenges like these you will want to preset some tethers so that you can get some points from objects you would otherwise be unable to get.
Hope this helps, have fun!