Be efficient with your slashes. You will be graded as follows.

Bamboo: The more of the board that is cut, the more points will be scored.
Slash: The less slashes used to reach your goal, the more points will be scored. Try to aim for larger chunks when possible.
Time: The faster you reach your bamboo objective, the more points will be scored. Move as fast as possible but do not hit any shuriken.