Colosseum Ultra Shortcut

Setup

By boosting up any of the slopes in the second mandatory arena, you can get on top where there will be a ramp facing toward the next race section. 

Angling the jump

Position your self just a little to the left, aiming to go around the ruined building. Boosting off of the slope will give you enough speed and height to get directly to the finish line. 

During the skip

Boost as needed to clear the gap, and wait before the finish line for the race to start. The flag won't spawn until the timer hits zero.