For me, "Oi! Lets Skate!" was one of the hardest challenges in the game. You need to air up to a rail (at Slappy's park) and do a tweaked handplant on it. You need to use pumping to your advantage and be pushing up on the left analogue stick as soon as you land at one side, then releasing it as you go up the other side to get a lot of air. However, what makes this harder is that they put in an A.I. character (Jake 'oaf' Brown) who crashes into you at every possible opportunity making this infuriating. You need to keep getting up speed, then as you go up the ramp hold rb and if you have enough speed you will do a handplant. When you do a handplant move the right stick in any direction to tweak it. Then land and the challenge is done.
A few points about thrasher challenges:
-to unlock them you need to have found 3 pools then go to the pool party.
-you meet the lady who drains pools for you by playing the thrasher challenges.