Quite funny, now endlessly respawning enemies, enemies with endless supply of grenades, bad friendly AI and bad game design in general count as a great example for a difficult game?
That's not actually difficult. That's just bullshit and badly designed. The player will just try over and over and over again until he is lucky and gets through the current part. Replay the same part again and he will have to try it over and over again. Unless there is an exploit to get through it. Just like Killzone 2's last boss before the patch.
A properly done "hard difficulty" mode should be difficult at first, but then get easier when you have learnt how to play properly and you shouldn't have to look for exploits, so that you can get around bad game design. Sure, it's pretty hard to design a combat game like this, when your enemies are using hitscan weapons for the most part.
Best example for well done hard difficulty: Demon's/Dark Souls. At first it seems really difficult. After hours of gameplay it will get easier because the player will know how to properly handle enemies. And in the end, the player can actually rush through the whole game within a few hours without having problems.
Another example: Metal Gear Solid2+3 on Extreme/Euro-Extreme difficulty for the most part. Sure, that's really difficult at first. But the more you know the bosses, the easier they will get. The game is absolutely fair in that aspect.
The "press button a thousand times within 60 seconds to survive"-part in MGS2's Euro-Extreme difficulty reminds me of those typical AAA shooters. That's not actually difficult, but simply bullshit and badly designed.
Just imagine the bosses of MGS2+MGS3 while you are having the typical AAA full health regen. They would be piss easy, because you could simply go into cover for some seconds and try over and over again. "The end" wouldn't be one of the best boss fights ever, but actually laughable. It would be boring as hell and you wouldn't have to learn the bosses unless they are able to kill you in one hit, which would make regenerating health pointless.