I saw something interesting, have the mage fire the fireball, then have both players turn on their shields, the fireball will go back and forth faster and faster and eventually the ninja will get blasted off of the screen.

yeah i have found that too, i think it is because movement is actually gradual teleportation each frame. as the fireball gets faster it teleports further and further each frame. eventually it skips past the shield and because the ninja has a smaller shield it hits her first im going to continue to update ( i will next add upward attacks ) but im not sure it will be worth fixing this considering how hard it would probably be and i would rather spend the time adding more features

Nice game! A simple (but maybe not so nice ) fix to that kind of problem is to limit the maximum velocity of objects. Another is to use getObjectAtOffset for a number of different Points in the direction your object is travelling, to "pre-detect" impact (and since the velocity and the approximate distance is known, a good Point of collision can be estimated, and the collision resolved properly, but this requires some math). I also found that if the mage is standing to the left of the ninja the fireball will slow down and eventually stop in mid air instead of accelereating if they Bounce the ball between eachother... :)