I found Yunalesca quite challenging. She has 3 forms and she is one hell of a fight! I mean if you get stuck as 3 people non zombified you are screwed. I was stuck for 5 months straight but the whole idea is too level up aeons abilities and to make Bahamuts Megaflare do at least 14 K or 20 K.
EXACTLY HOW I FELT
I even summoned Aeons to battle (I wouldn't normally do that, so~) Who in hell would think that you need to be ZOMBIFIED?! I wasted quite a lot of effort, ironically, Zombie-Warding/Proofing my characters
Hmm, thinking of the whole FF series that I've actually PLAYED (which isn't quite a lot, but aim is 100%), Cloud of Darkness in FF3 was the most challenging as of yet. I was thinking that the first fight was it, but NO~ you get sent to find that warriors of dark/light (can't remember) and the dark crystals and once you get to that stage, you either teleport yourself out of the tower to save, wasting all that effort to climb the tower, or go on to the 4 minor bosses and the last boss, which IMO were hard.