Oh man, this thread saved my butt. Thank you delgado! (sorry for reviving this thread. I think it's very useful)
For those that cannot download it, the trick is to set a global variable for shooting on the left.
the global variable is set to 0. when character is facing left is set to 1.
Have a condition that states, when global variable is set to 0, projectile is shooting to the regular right. when global variable is set to 1, have projectile set to 180 degrees, and it shoots on left when character is facing left.