Not a particularly hard boss fight. Some points:
-Hang back on the back ledge with the two turrets, and not out in the open. The main cannon only fires inside the open area.
-Missiles will not reach you about 90% of the time if you stand behind a wall on the upper ledge at the back. Only had one time where I had to move, and it was easy.
-There's a homing rocket launcher above the middle ramp to get to the back ledges, but on the middle ring thing. You can jump over to the rings from the cannon points on the back ledge. Helps out a lot for taking out the flying minions.
Other than that, just keep shooting at it, and eventually you'll kill it. Hard is basically the same, although you'll want to watch out for the main cannon - if it hits you, you're basically dead in one hit. Just hang out up the back and it's yours.