Game: Resonance of Fate
Boss: Rebecca/Sulivan on Zenith mode (5x enemy stats)
Conditions: Using all 3 characters.
Two bosses to fight at once, if you get hit one time by almost anything you will be dead. While this fight is super hard, theres one strategy (the only one I know of that actually wins this fight) that makes it quite easy. I've got a youtube vid of it, I dont think any other strategy would actually work against this boss.
Game: Star Ocean 2
Boss: Unlimited Indalecio/Gabriel on Universe
Conditions: No Bloody Armours (all damage hits you for 0, why is this in the game), no Algol Shield (1 hit kill chance).
Ok so you have 9999 HP. He can use magic that will kill all 4 of your members in one hit at full health without a bit of luck and defensive buffs. This dude moves so fast around the map you might as well say hes traveling through time. Not only that but he casts magic continuously and he can cast them WHILE MOVING. He has an insanely high daze resistance and has 3,000,000 HP. I also forgot to mention that he casts magic in about 1 second compared with every other enemy that would normally take upwards of 4-5 seconds.
Game: Castlevania - Portrait of Ruin
Boss: Dracula + Death
Conditions: Hard Mode Lvl 1 Old Axe Armour
Super super super hard. Like wow this is hard. The main thing that makes this hard is that you have to be so consistent. 2 hits and you die, you do 15 damage per hit and the lowest hp boss has something like 6666 HP. This is the only boss I can think of (except the next one) that I have not been able to beat on the hardest mode.
Game: Final Fantasy XIII
Boss: Orphan 2
Conditions: NCU (No Crystarium Usage) and No Elixirs.
Think you have felt frustration? Sorry, but unless you have tried this, you dont know what it means. I'm serious, like really I'm not joking, theres a reason why under 10 people in the world have completed this. This boss has 2 forms, one form (Light Form) he heals himself continuously and can attack, the other form (Dark Form) he puts all kinds of status effects on your team while dealing some pretty damn hefty damage. He alternates between these 2 forms every 2 Minutes and 41 Seconds and gives a little message a few seconds before it. When switching forms he hits you with a powerful attack that deals some sort of % of your HP and his chain gauge gets completely reset. Without Sentinal active (you cant switch during his attack, its one of the few moves where u cant, you have to switch before it) your leader will die unless your at or near full HP.
It gets worse. You have 33 Minutes and 30 seconds to kill this boss, because at 30 Minutes and 30 seconds he will cast doom on your leader (which lasts 3 minutes). Don't forget that this boss heals itself continuously in its Light Form and you have to spend most of your time healing/staying alive in Dark Form. Oh you think its bad now? It seems you dont understand the fact the he will randomly cast a ONE HIT KILL MOVE ON A RANDOM TEAM MEMBER. Yes, that means that if he uses it on your leader then its time to restart your half an hour battle. He has about 7 million HP and provided the gods have blessed you with success thus far, when he hits 2.7 mil he goes into a dual Light/Dark Form. Thats right, now he heals continuously AND he does huge damage on you.
He also gains a new move that drains the health of your team and heals himself ontop of his normal healing. He gains another move that for some reason completely misses you sometimes, but if it doesnt then it requires a full HP sentinal otherwise you will die. He will still use his "switching form" move, although he no longer gives you the message (you will need a stopwatch handy or your finished). So somehow you have to juggle surviving his super damage while at the same time out-damaging his healing before your time limit runs out all the while hoping he doesnt instant kill your leader in a 33 minute fight. If, by some miracle, you managed to achieve all this, you should finish the battle with about 10-20 seconds left on your doom timer. Its basically impossible to do it any faster. Easily the hardest boss challenge I have ever seen/attempted. Despite full days at attempting this, I could never do it.