I'm going up against the super mutants and I cannot figure out how to fire the minigun (on PC).

Well I fire mine off with "Flight of the Valkyre" playing. Sorry about spelling.

Sorry though I took a break from the game so I don't know how its actually done

Hit E button?

On pc its just to fire it like any gun with left mouse button, with the downside that aiming removes the crosschair but as you have unlimited ammo its just to walk the beam into targets.

It's the same button that you always use to fire. Left mouse on PC and whatever the corresponding button is on console. Presumable the right trigger.

This really bothers me. I don't know who thought it was a good idea to remove the reticle when zooming in on the minigun, but they were wrong.