The 'true' final battle of Devil Survivor 2: Record Breaker. Not only the most graphically intensive sequence in the game (reminds me of Giygas except less evil-looking and it doesn't speak), but one of the only points in the Triangulum Arc I found legitimately challenging, though part of that may be because you don't get any kind of rest period between the three phases and I was running low on everything by the end.
What made it stand out for me was that the various components of the boss each have powerful abilities that they share with each other- infinite range, infinite movement speed, double the number of turns, and double damage. but after defeating each portion, your party gains all of those same abilities... and you will need them to stand up to the final phase.
The last few turns when I killed it were insane- all my teams had revival and healing skills and had to use them every single turn to get someone else back up as well as attack the core to reduce the number of turns it got (left unchecked it gets about 1.5 turns for every 1 your party gets, and its main attack hits everyone for big damage and can't be negated or outranged). By the end it was just my main character's team left barely standing with 200HP... but I did it. Polaris wasn't necessarily bad, but this was a true finale.
The only other time I've heard of a fight like that was a boss in WoW where you would beat adds to earn access to a 'super mode' with enhanced abilities, HP, movement speed and other major benefits, because it was almost a requirement to beat the boss.
