it's an ok idea, but there's no penalty for hitting the wrong square. there's nothing preventing the player from holding A/D and spamming spacebar.
make a more complete game (starting with making the shurikens look like shurikens]
best of luck improving.
Author Comments
AD or left/right to move the sword
Spacebar to change fire/ice
fire sword kills ice shuriken's and ice sword kills fire shuriken's
protect the heart
restart if too much lag