So I finished hard mode. 01:53:03.
That was a nightmare. No, seriously.
Probably only due to my completition obsession, since I could've skipped the hardest fight (two Annihilators - the hardest regular enemy: constant stream of bullet, only one weak point, super pellets, normal shots, split shots, random move pattern, pretty big, has a thing for pressing you against the wall).
Don't try this unless you defeated Ballos.
And there goes Annihilation difficulty. 00:46:16.
It's only a short sidestory quest which takes place before the main content - and the game doesn't break continuality:
You don't get any upgrades for the whole time, so you're the same as at the beginning of the main content mission.
That's the main difficulty. Also, you fight all of the most hard and annoying enemies, with ABSOLUTELY NO PROTECTION
AND A WHIMSY LEVEL 1 GUN.
The last boss is hardcore, but with a learnable pattern. Took me less attempts than the two annihilators on Hard.
(Still, this was really, REALLY hard.)