Not hard, but biased!
Looks nice, feels nice. But the turrets are too hard to use and the targets fly a tad bit too fast for a casual person. Also, it would be nice to know what level your turret is, what is the max level for it and how much experience it requires.
The best strategy I found was to use a machine gun and sniper/lightning gun with a couple of damage buffers. I used machine gun 90% of time and the other weapon to pick off some annoying bigger enemies.
But that was with easy.