If you finish a level with 11 times more shurikens than lives, you will get an extra 30'000 bonus points.
In round 4 - 2, on the first platform jump and shoot and you'll reveal a power up giving you 2 lives on the platform across from where you are. Pick it up and then die. Repeat the process to get as many lives as you want.
To get unlimited shurikens set the number of starting shurikens to 0 in the options menu. After about 20 seconds the 0 will change to an infinity symbol, giving you unlimited shurikens.