A good start position for further development, but needs some work.
1. The most important: at the current stage the game is creating impossible patterns - or rather there are no patterns, so it is creating impossible situations. My ninja just died, because I deflected 1 shuriken from below, one from the left side, but at the EXACTLY same time another was coming at me from the right followed by an arrow and one ninja approached from above. Is it avoidable? I guess it is a rhetorical question.
2. I guess we got an omnipotence paradox here: the developer created a game that he cannot beat. A challenge for the developer: test all of the medals, pass the game and by doing this prove to everybody that the game is doable at the current stage :-) If not - rework the game. Re-balance it.
3. A small suggestion: enable restarting with for example the space key, or R, or enter, but enable a restart with the keyboard.
4. Adding a Newgrounds scoreboard would be a good idea with separate boards for kills and time survived.