One way to get a fairly high score (but still not nearly as high as the one
mentioned above — I have no idea how that's possible) is to push the
dummy's foot directly into the step he is standing on. This causes him to
collapse, and the entire weight of his body falls on his leg. Sometimes he
stays stuck in this position, and you can rack up very high damage points
(accompanied by very satisfying crunching sounds) before he slowly starts
rolling down the first step.
How to do it:
It will probably take a few attempts before getting a really high score, but your patience will be rewarded.