Arena Battle

This is the simple arena example. :D
How to play:
There are four weapons which you can use. Three of them you can buy in the shop with rubies.
You get rubies by killing waves of enemies.
How to launch the wave:
Just exit the shop. You can't buy anything while the wave is alive.
Derivatives are allowed so newbies can look how the code works.
That's it. Have fun. :D