Alright, here's the thing. You're literally at the end of the game now and this'll be your last chance to train, collect any Items you've missed, Monsters, etc. Just remember, some of the Monsters you could be missing could be from the Jade Passage and Pandaemonium, the final two dungeons in the game, so don't panic just yet. Also, the only real trouble you'll probably have with any Enemy is the damn Goblin Prince. Check Section VIII. Monster List for more on him. Other than that, you should be able to find every other Monster rather easily and if you've been following my Walkthrough, I've listed all the Monsters in the area, so you should've hunted them all out to make sure you fought them at least once. If there was something you missed due to a Treasure Chest with a rare Monster inside, like Fallen Emperor and you can't go back to it, then you're shit out of luck, sorry.
Either way, the only reason to get 100% of everything is for the hell of it, really, mainly for the complete data and all. As for the Items, again, if some areas are closed off now, then oh well, you can't get them. Should've gotten everything the first time through. Though, for the Art Gallery, basically, that's filled in as you get more Monsters in the Bestiary, so don't worry about that. Anyway, enough about that. For training, I suggest a minimum of 2,000 HP for everyone, with mastered weapons/Shield at Level 16, (This will NOT take you that long. Just cheat to do it and to make it go faster, Equip 2 of the weapon you want to train in your hands and watch how fast you gain 1 Level. Same with Shields.) have your spells at Level 10+ and for MP, 350-400+ will do fine. I strongly suggest those levels, as you'll really need them. If you want to do more, by all means, go for it, but that's just to get you by.
Though, ONLY train the spells you will need to use and don't train the ones that don't need it. (Life, Warp, etc.) Also, I suggest doing thing around Fynn, mainly since the Battles here WILL get you a TON of Gil like I said before after the Mysidian Tower. So, yeah, whenever you're ready, make sure you buy… oh, 5-10 Elixirs, which I'm sure you have the money for. This will just seal the deal on you beating this game. Do NOT buy Ethers, as they are useless and Osmose works so much better.
When you believe you're finally ready, go inside Castle Fynn and talk to Princess Hilda and she'll tell you about the creatures who flooded the world through the Jade Passage. Memorise Jade Passage and then ask her about it and she tells you it's near Mysidia. So, head back to your Airship and literally, from Fynn, head south until you pass the Mysidian Tower and once you reach the next large mass of land, head east towards the end and look on the land for a small pond like thing. This is the Jade Passage. Land near it and when you're ready, enter it. (Don't be afraid, you can leave whenever you want, it's not like you're stuck here forever. Same with Pandaemonium, so don't worry.)