This needs to be done in a SINGLE combo. The three tricks to remember are to
1) Save up your shockwaves. Use all four of them in one combo, and only use them when your combo is about to expire. You can use them with
.
2) You can use nitro on the yarnball! Use this to keep yourself moving towards high ranking areas. You can activate nitro with
3) Aim for the billboards on the sides. They're worth a decent amount, will keep your combo going without needing to sacrifice your explosion abilities, and will increase your multiplier. Kevin also suggests fences and gates, which are also fantastic for combos. A good general tip is to stay at the side of the road if there aren't any cars immediately in front of you.
If you have anything to add or any complaints, please comment! I'm happy to make this solution as helpful as possible.