I think the hardest, or atleast the most time consuming page is the sythesis notes. Gotta find 30 of every gem to complete the "purchase in shops list" OK not every one, and not even 30 for some. Just gotta get 30 shards, 35 stones, and 20 gems. mythril, bright, energy, serenity, and all crystals arn't on the list.
The final colliseum cup also quite challenging. I tried it at level 84 and was no match for it. Only cuz of the part where you have to play by titan cup rules, that summons only and 500 hit points system sucks.