24. Hardest bosses for me in LO were the Grigan, Flare Worm, Kakanas Tank on disc 4, and probably the Arthrosaurus as well but that was due to the time limit on the minigame there. The Grigan in particular however was a real pain in the ass. Was gonna say Gongora too but that was about as much of a fight as with the first time you fought Yggdrassil in TOS.
Tried facing the Experiment 1 mini bosses at Grand Staff but the fact that they self recover around 700 HP every turn when the most I can inflict on them is around 1200 points of damage with Jansen, Ming, and Sarah all using Flarus on the SOB and not counting what damage Kaim and Seth can muster too is pretty detrimental to say the least, especially when they spam Shadowrus and start buffing at some point. Then again I might be underleveled right now due to being around level 46 across the board and all.
The Barthandalus/Orphan hybrid was what I was referring to, my god I hate that bastard so much still to this day.
Most of the secret bosses weren't too bad except for the Lunarsaur, oh my god was that thing hell to face, Dark Bahamut was pretty bad too but Lunarsaur gave me the most trouble by far.
Zeromus wasn't too bad until it started spamming Big Bang and Meteor every other turn, Big Bang I could prepare myself for it was Meteor that ended up screwing me over the most.