This may fall under the umbrella of ''Hardest Boss Fights'', but this thread is for posting about your experiences against those bosses that just would not die, whether it be from cheap gimmicks, ungodly amounts of HP, or just plain old hatred from the developers.
Before I post some of mine, I tell you what inspired me to create this topic. According to Megagames, a brave guild of users called Beyond The Limitation tried to take on Pandemonium Warden in FFXI. They lasted eighteen hours before finally throwing in the towel, due to him having twenty forms to switch to. Whenever he does that, he regains all of his HP in addition to spawning eight powerful monsters. Ouch.
Okay, some of my experiences, though not quite as extreme:
1. Taboo Sho Minamimoto (The World Ends with You)
This guy is annoying and takes a while due to two factors: One, he takes 75% less damage then normal enemies. Two, he almost constantly teleports away from you when you go in to attack. And all that together and you get a very low damage output. I had to rely on the top screen partner to deal damage until I could throw together a powerful Fusion, which took quite a while.
2. Star Magician (Golden Sun: The Lost Age)
This one is irritating because he keeps summoning minion monsters that refill 9999HP as well as erecting barriers to reduce damage to pitifully small amounts. They also dilute the damage of your powerful Psynergy, as you can't pile them all onto one character and one alone.
3. Wedigo (Final Fantasy 5)
He has four clones that he constantly shifts between, and there's no real way of knowing which is the real one, resulting in a very protracted fight.
4. Gollum (The Return of the King (PS2))
The gimmick here is that you need to force him to the edge of a cliff and perform your finisher to knock him off. The problem is that it's almost impossible to budge him in any direction with any of your attacks. My cousin and I spent close to three hours fighting him on a single life until we barely won.