I really hate to say this, but the Boss in Final Fantasy X, named Seymour Flux.
This boss really gave me a hard time, there was a time where I almost won, but Total Annihilation, well, you get the idea.
It was later that using Shell will help drop the damage by half during T-A, but I didn't know at the time.
I was wondering if I was really underleveled, I tried using a Aeon's Overdrive, and hit about 9999 damage, but the question is... how? Used Bahamut's Ovedrive, and hit well 20000+ damage, now... can someone tell me how am I hitting this much damage? (And no, this is NOT a joke.)
I did view a video to see if there were any ways to get pass this boss, in one video, I saw Bahamut hitting around 8000+ with it's Overdrive, after seeing the other Aeon's hitting 9999 with their Overdrives.
And no, I don't have a cheating device, if I did, I wouldn't be using Aeons, and would just use the characters and hit well over 99999's each time.
Now... I have to deal with Yunalesca. Her 3rd Form... Argh, Mega-Death... which kills your whole party if not one of them is in Zombie Status.
There are other Bosses that gave me trouble, but I don't want the make the list big.