What was the hardest boss battle you've ever seen in any game?

Rugal from Kof95. You had to beat a flame throwing guy before him and your REMAINING characters/health had to face the actual boss.
He had A LOT of invincibility frames, a move (genocide cutter) that could one shot and your best combos hitting him for (at most) 5-6 percent of his health.

I played on an arcade machine (how lucky I was to have kof95 at the game center) and spend all my money as a kid on that one.
Ah, no matter how much money that machine sucked from me I was never able to beat him, and I "ragequit" for like a week after he beat me with ~10% remaining health.

Found a video with the fight (non hacked or else) where a guy gets away exploiting a bug on the AI: https://youtu.be/6Y8i_J_OcY0?t=1245 you can still get grip on how powerful Omega Rugal was.

This is not tie to single game, its just game mechanic most commonly seen in MMOs. Getting hit, most often its one-hit-kill move, from no-where with invisible attack and not telegraph of any kind. Games where bosses difficulty is measured by your gear level.

Other items from list that are rage inducing are QTE and idiotic AI of escorting NPC.

Almost forgot, cheating boss AI...

Funny how "Protect NPC" is on top of the list. I mean, yes, escort missions are generally terrible, and I loathe them. But vulnerable NPCs in boss fights for me are actually a good element to force the player to do action and not just camp/potshot or circle-strafe all the time… because lets face it, most bosses are either pure gimmicks or can be killed by the Almighty Circle-Strafe.
Of course hen said NPCs are programmed to walk directly in the line of fire instead of covering, that's another topic.
But really… if you think about it, the combat in, for example, Mass Effect (or in most RPGs where you cannot control the party) is pretty much this: fight with NPCs to protect constantly. And to my knowledge, people like it.

I personally hate when suddenly the action is stopped and the game drags you into some bullshit QTE minigame. Not to mention that since I'm playing on keyboard, half the time I'm suddenly forced to press a key in 0.000000000000001 seconds that I never even use in the normal game play.

But what I'm missing is two things.
One is when the game decides to change all the rules for the final boss, because reasons. Like in Trine, when suddenly the whole game mechanic is changed. Or when the last level and most of the boss fight in Max Payne 2 turns into a platformer.
The other is when the boss can hit you, but you have to destroy half the level or push a trillion switches to be able to fight back. Plus points when it's cyclical, so you have to do these rounds again and again. In platformers I can live with it, since it's one of the few mechanics they can use in a boss fight, but apart from those…

Ragnaros heroic

World of Warcraft - Firelands

If anyone have ever participated in guild race for kills, then i dont need to explain why and what. It was THE most difficult encounter ever made (before nerfs ofc).

American Mcgee's Alice
Voracious Centipede
The hardest bossfight I remember. We tried to beat the guy while drunk, swapping seats on death etc. It took us 3-4 hours, but probably it was so hard because we played while being hammered.

Regeneration health.

I can deal with npc protection, 1 hit kill and all of that but if I spend 20 minutes removing 3/4 of the boss' health just to watch it refill 'cause I didn't kill him quickly enough, I lose my shit :D

General Raam
Gears of War

1. Sahelanthropus
2. METAL GEAR SOLID V: THE PHANTOM PAIN
3. Nope, 7th try and still no.

The most problems I've encountered (when I was younger) with bosses was with dual bosses most of the time. Agni and Rudra (DMC3) or the second to last Boss of PoP:Two Thrones for example. Having 2 diffent guys attacking you (and in the caso of PoP having to win a Quicktimeevent so the fight progresses) on top of a questionable camera perspective really let me ragequit that one for I think it was 2 or 3 years before going back to it and doing it first try.
Now I don't have so many problems with multiple bosses in one fight anymore. One-Hit-KO though will always be a problem since it doenst give me space to try out and learn smth for my next attempt and that sucks the fun out of everything - being not able to progress in any way

1. Mega Satan on hard(secret boss)
2. Binding of Isaac
   3.Havn't faced him yet

Ragu o Ragula from the old Wild Arms 1. WTF to the max.

1. Pandemonium Diablo
2. Diablo 2
3. He has some resistances, high dps, immunities and has lethal combos of spells. you need some serious gear to farm him =)

• Sir Alonne
• Dark Souls II - Crown of the Old Iron King
• Couldn't beat it. Tried everything but the bastard is too fast for me. I beat Ornstein and Smough on my first try but I can't do sh*t to this smug bastard. I heard that it's a bug that he moves so fast due to the fact that I'm rolling on 60 FPS instead of 30 FPS (how the game was designed, for the consoles) but I never tried to limit my frames. Might give it a shot though.

Last boss – Drakengard 3

Imagine, if you will, a game series focused on story with bad combat layered on top, got it? Right. Now take a flower mix it with a few animatronic statues of girls and a catchy tune, then add in through fire and flames on expert mode and change the rules so if you fail even once you have to start over.
What are we left with? The last boss of Drakengard 3, But I didn’t mention the secret chemical X, but since I like you guys I’ll tell you what the X stands for: Extreme pain, misery and torment.
This boss is so difficult that the most commonly accepted way to beat it online is to take video that shows you how to time your button presses and start it at the same time the song does, which in itself is a right pain the knob, but honestly? Using the video is at least one hundred times easier to beat this boss unless you’ve studied music theory.

http://i0.wp.com/www.dangerdolan.tv/wp-content/uploads/2015/03/22.png

Source:
http://www.dangerdolan.tv/top-15-hardest-video-game-bosses/3/
You can check some of the other boss in this artical too.

I Agree with this artical whole heartly.. So i didn't change a word of it.. took me around 100 tries to beat it.
And i broke 2 remote controler in rage.

Here is a video for that boss..
https://www.youtube.com/watch?v=IpgukXzA_-E
looks easy right? try it yourself...

I usually get destroyed by any semi-hardcore bosses since I can't handle that kind of fights (or games).
But the hardest I did ever beat is Ballos from Cave Story. Granted, I had to play in Easy Mode to handle him. I did complete the Normal Ending in the original difficulty setting though, which is by no means a walk in the park either.

(SPOILERS FOR CHRONO TRIGGER)

1. Chrono trigger

2. Lavos

3. I will chose Lavos from the end of the game. I just couldn't get him damaged. He has 3 forms, regenerating health and a few almost one-hit KO's. I spend 2 hours on fighting him with no pauses, and still failed ;)

I went with others and it's fighting a world boss that 2-3 top clans to fight in a PVP zone when the other 2 races from the game crash your world boss run and wipe you in PVP.

Game was RF online back when code masters ran the servers for EU and USA.

Some events in F-Zero GX were really hard on normal difficulty. Not to mention extreme difficulty...

M.A.C.E from intrusion 2. and I haven't given up, I'm just on a break

1. Queen Larsa
2. Mushihimesama
3. Makes most other bullet hell encounters feel like bullet paradise. E.g. https://www.youtube.com/watch?v=quOPbRFiJa0

1. Zero
2. Drakengard 3
3. i nearly gave up was unique rythme boss (when game is a hack & slash) that you need endure for around 7 min (one mistake mean u have to start it again so is like 1hit KO) camera angles and sounds mislead your vision and rythm. For worse it, everytime u fail, start one cutscene losing a lot of time on each try (because u gonna fail a lot of times).

Protecting a npc who does the most foolish things and gets itself killed all the time. Oh how I hate that..
Can't remember which fight I thought was the hardest of those I've had though, but yeah..

Edit: Counting some random people in online multiplayer games as npc's as well in this regard, since many of them act exactly as foolishly as the npc's themselves, at least to my own experience when playing mmo's before.

I don't really remember specifics, but I could never get past many bosses without using a Game Genie in "The Guardian Legend" for NES. Sometimes cheating can make certain games more enjoyable. I haven't tried playing the game without a Game Genie since I was really young, probably since I was 10 or 12 years old (about 17 years ago). Maybe it's not as hard as I remember it. I'll have to hook up the NES one of these days and try it again.