Each time you jump between platforms you fire your weapon.
If you jump left or right, you fire up and down.
If you jump up or down, you fire left or right.
Enemies will always hover between platforms so they can be hit. (They are just nice that way.)
There is a bug where some shots won't hit specific spots (downward jump specifically misses often).
You can still hit those enemies by moving to a new platform and performing the needed jump.
Hope that helps.