It is Recommended to have a maxed out character for this but it is unnecessary. I achieved this in just over the 1 minute mark inside the halfpipe, and ONLY inside the halfpipe.
It's really easy actually. STAY IN SWITCH!! It doesn't really seem to affect your characters ability but it multiplies you score. In the upper right hand corner of your screen, you have a skateboard. If it doesn't say switch on it, pull your right trigger and the boarder will flat ground spin to switchfoot. every trick you do in switch shows up YELLOW and has increased point value. Also, if while on the halfpipe, you find yourself with normal footing, pull the trigger just as you land or before you jump to "Revert", switching your footing on the board, and adding an extra trick to your multiplier. It's also a good idea to vary your tricks a bit as the value goes down with each use. Also, I recommend not using stalefish or madonna as they seem most likely to throw off your landing.
I got this by continuously going up spinning + fliptrick + grabtrick and hold grab til last second. Add a revert on the landing or takeoff and i was getting 10k-20k per jump.