Hello! So sorry for the lack of instructions, hopefully I can clear this out:
You don't have your own bullets, instead, you absorb bullets from the boss using your Vortex (left mouse button). Hold this while getting close to the projectiles from the boss and then when the lines align, click the right mouse button to shoot at the boss! Also a tip: you still get hit even with the Vortex if you collide straight on with the projectiles, so make sure to be just close enough to grab them but not too close that you get hit!
Hope this helps!