In this level, coming from the very first THPS, there are some easy tasks to do. To get the Sick score, you can do various things. You can either do combos with grinds & manuals, or do big airs in the half pipe. I did the latter and managed to get 10,000 to 20,000 points each time I did an air. Just remember to hold the button in the air, add some spins with or and release all buttons just before you touch the ramp again. Landing with perfect rotations grants you an additional amount of points (you'll get the word PERFECT in green when you land it), but landing with not enough rotation can extract some points for the air or combo you just did (you'll get the word SLOPPY in red when you land it).