Imagine playing a terribly generic game like this for 20+ hours, then getting to a final boss that completely and utterly destroys you, with no possible way to defeat it. That's what this game is on the hardest difficulty...a horrible, broken waste of time. I don't normally hate games...in fact, I have tons of bad games and love a good portion of them...but this one is a very clear exception.
EDIT: FYI, the reason I have 20+ hours into this game is because, on the hardest difficulty, it can take over an hour killing some bosses, since you will completely run out of ammo within a few minutes (even if you started with max ammo) and have to spend a short eterninty shooting the bosses with your weak, inaccurate, infinite-ammo sidearm until they die (and if you break cover for more than a second during this time, you die and have to start over). The endboss is the same thing, except with a 5 minute time limit (and the boss isn't even on screen for most of that time...and you have no cover...and you are surrounded by regenerating flying turrets. Literally impossible to beat).