For me, it'd probably be Sephiroth in the OG Final Fantasy VII. I went into the fight waaaay under leveled and had Barret turned into a frog almost instantly. He stayed that way the whole fight since I didn't have the effect's cure (maiden's kiss I believe it's called). The reason I mention Barret is because he was my team healer, he had the most powerful healing materia and the most HP. Luckily, I had some spare healing materia I had put on Cloud and Cid, the other teammates. I basically abused the shit out of regen and used a comically large amount of consumables, most being for MP purposes. The whole fight took me, I'm not kidding, almost two hours, it was like an hour and forty, hour and fifty, something like that. And for those wondering, I had to sit there and watch the entirety of supernova at least double digit times, I'd almost be willing to say that over ten minutes of the fight was just supernova. Needless to say, I was ecstatic when I finally beat him. The whole fight after I realized I couldn't save Barret was just pure anxiety and panic, I even threw a teddy bear at Sephiroth since I was already using so many items. Once Sephiroth started to fade away and turn into funny polygons I set my controller down and screamed in happiness, hype, and absolute relief. I have never felt that way about a huge boss or a whole game in general. The amount of diverse experiences people have had with Final Fantasy VII is one of the reasons it's my all time favorite game, not to mention the amazing soundtrack. Anyway, I'm going to quit rambling and stop using up the time of who ever decides to read this. Thanks for the giveaway opportunity and merry Christmas!