Just finished the game, here's a few simple tips on who to use and where:
For rear wheelies: Mr. C
For front wheelies: Letter F
For boost multiplier: Sir Colin
For speed: The Wheel
For airtime: The Man With No Brakes
As far as the boss battles go, upgrade your speed and acceleration as much as possible to start. What I did to beat them was simply to hold forward and accelerate (right key + w) and try to avoid falling.
This will cause you to hit the rocks but you'll be able to avoid the enemy by recovering quickly.
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