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.

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.

Slash
The less slashes used to reach your goal, the more points will be scored. Try to aim for larger chunks when possible.