this could be any, from story or random encounter.

im kinda biased because its the fight i am stuck on, and also forgive me btu i amusing PSX translations because thats what i played it on
anyway that fight is against Balk in the necrohol of Mullonde. i read up that right after this is hashmal and finally ultima, however the hydras are too powerful even with TG CID!!! on my team.

other hard fights include, Dorter slums, Some drained lake with ghosts in it, the first mountain where you see summoners, and the fight against gafgarion in the Execution grounds.