Alma from Ninja Gaiden. Never have I been stuck on a boss for more than a few hours. This one had me smashing my controller for almost 2 months. Now that I go back and play it she's actually not that hard. I guess I just really sucked way back when.

From what I have played definetly Radec and his personal guard in killzone 2 (hardest difficulty).

In the robot plant ? I don't remember very well

This game was very good but severely underrated. SP was very good, since the difficulty level actually added in the level design instead than just the usual 'bots have higher HP and insane firepower'. MP was fun as hell, with a lot of customization. There was also co operative mode. Timesplitters had a lot of features that are considered as 'revolutionary' now ^^.

Boost Guardian. Hard Mode. Metroid Prime 2.

The difficulty is way out of proportion to the rest of the game; the random element to the attacks and pickups is taunting you; and the whole setup is just unfair with constant HP drain no matter what you do.

Well, I guess there are a couple of Touhou bosses that can be classified as "hard".

Emperor Ing in Metroid Prime 2: Echoes, never beaten him.

going by that definition....last boss of fable 2.

but a real answer, the speed ball boss of metroid 2.  never finished the game because of that boss.

Omfg, that entire level on hard was impossible. It took my friend and I 2 years to finally beat the entirety of TS2 on hard mode, and it was all because of that stage. We finally did beat the game last summer.

The final boss on Dead Space, on insane. I did'nt think that I was going to beat it, and was about to give up when I finally beat it. That game is so intense.

Saga Frontier 2 - Had 2 endings and not alternate ones, the game had two stories.

One ending was a Boss the other was a Boss as well but based on the strategy component of the game.

I still haven't beaten that game... The game for those who don't know feature a LP system, that your character has a limited amount of through the entire game. (Some characters grow this) The LP doesn't change with levels. Your HP shield your LP so your character can have 0 hp and still fight but if they get 0 LP they die for good. Fire Emblem Style but without spare characters to replace them.

So of course you had proc where a monster would deal direct LP damage even if your HP is high and the Boss loved to proc.

But the worst part was that the boss not only hid hard and could proc but that it had at least 5 forms at least.

The second ending was nothing short of a pain as well, even on game faqs it used to say that it was impossible to end this and if you have you probably live in Japan. (Japan had the PShandheld device) (The very first one not the PSP)

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Recently... well Monster Hunter had some freakn tough ones, DevilJho is just plain fun while Alaterion is just annoying. IDK about todays games.