Nikki Rydia
The Hell Wyrm wasn't all that hard to me, it just took forever to kill him. All I did was berserk two of my characters and I controled the healer and stayed out of the fight. I had my gambits all good to heal slow, so it was just a very long battle to me...
The hardest boss that I can remember would have to be Lady Yunalesca...it was the first time I was fighting her way back in 2003 and I couldn't beat her! It took me forever to make up a strategy! O.O
Hell Wyrm i thought was easy also though i like your method better than mine... But i had excallibur by this point extra damage... yey ^^
Yiazmat i found quite hard and i have yet to challenge Omega he wouldn't fight me for some reason
oh and Gilgamesh part 2 had me screwed for a while until i just use Oil on Enkidu and burnt him to a crisp.
Oh my gosh yes!!! i just remembered that fight now...
I got sent to the slaughterhouse so many times in the end i made it to round 3 and had Yojimbo use Zanmato...
Evrae = 3 i hated that thing.
Dark Yojimbo I can beat him but i can only find him 4/5 times.. i chase him to the back of the cave and Tidus starts panting and yeah... I want to kill him so i can see Penance.
Seymour Flux... is that the one on Gagazet... damn that one pissed me off had to Yojimbo that one too... well actually i was too lazy to strategise.
also FF X-2 Angra Mainyu that thing is hard...
and that chaco thing... is it called Chaco? its a basilisk type and its in that lower bevelle place before Trema..
FF VIII - Omega weapon... Damn near impossible for me. ^^;
But then bosses level up with you on VIII so they can be pretty easy and skill limited depending. x.x
FF VI - In all honesty the hardest boss for me in this whole game is that flame eater in the burning house... ^^;
FF VII - Emerald weapon untill i discovered a way to make sure he does 1111 hp damage and have everyone go into lucky 7
FF IX - Ozma..... >.>