This shouldn't be too hard for you to accomplish, especially if you made it through career mode. I would suggest using special tricks, especially if you can combine it with a few other tricks to boost the multiplier.
Either hit the rails around the level a bunch of times, or spend some time in the half pipe. You won't be able to revert into manual, so you won't get huge combos going, but if you mix a quick flip trick with a decently held grab, you should be able to get fairly decent scores from the small combos.
If you're going to try the half pipe, obviously increasing your air stat would be a good idea so you can hold those grabs longer to really milk the points. Just remember to switch up the moves so you get maximum points.