I genuinely had nightmares about this one as a kid. Fighting this evil guy while your girlfriend gets crushed to death in the background is brutal. The maze level before him sucks and you need to learn it and avoid damage to keep the damage upgrade so you can kill him in time... I don't think I ever managed it as a kid. The boss himself isn't so bad though, really.
It's a boring answer but Malenia is probably the hardest boss I've fought in a game. She's BS. Besides that I'm not sure.
This.... THIIIIIISSSSSSSSSSSSS jesus christ even years later I see this image and I just fucking hate that stupid fucking music.... I mean origins has made it even worse but dear fucking god that's is not a good memory.
Now this is hard because I feel like I have fought harder things in the past but as far as something that has recently stuck with me that made me think "man this is utter bullshit who designed this?"
In the end I had to look up a guide and essentially "cheese" him... the only way to win is effectively not play the game by the rules you have used up to that point and just lock down the guy and not let him phase shift twice ever, essentially you avoid letting the battle playout and make sure to kill him in under a very small time window... its very unfun and was the lowlight of the game for me... although it was nice to get the extra trophy for beating him after so much BS.
You can beat him with letting him phase shift, but he requires a completely different approach than any other boss. The only way I could beat him was with controlling Tifa, evading his moves very specifically (the stab you need to leap over, etc), and abusing aggro. Then pelt him with arcane ward casts in phase 2 and back to Tifa in phase 3. But you need to get his damage very low in phase 2, otherwise he will eventually one shot you.
As long as you don't take too much time he won't do immaculate end. Without that it is kind of a fair fight. But its just a weird one because I plays unlike any other (having to do perfect evades to pressure him, I guess the Turks have a similar gimmick).