Sheer insanity...
At first, I thought it kinda sucked. Took too long to get points, I thought. Then they came more easily, and I got into it. Then I plateaued and started getting bored again. But, before I shut it off, I gave it one more try, and got into a good "kick the dude, kick three ninjas, kick the dude, etc..." pattern. The resulting 126 MILLION mad points from that ONE RUN was enough to buy every remaining upgrade. DAMNIT, that was satisfying. Major props, dude.