To be fair, the 4th/5th skill trees are pretty fantastic... but some are a lot harder to get than others. Melia and Dunban's I know are actually very easy to get - they involve little quest chains in the Eryth Sea and Valak Mountains that are really easy to complete and that's it. Melia's in particular has some amazing abilities for tanks and non-tanks (specifically Seven), and Dunban's is amazing for anyone who wants to play dodgetank, including himself. However, all the others (I think) require you to develop a very high affinity with an area - which means grinding out a lot of quests.
All the 5th skill tree quests are postgame-tier difficulty that has you fighting level 90+ enemies to achieve it apparently so I haven't got any of those @_@
On an aside though, while I love this game to pieces, trying to get Advanced Art Manuals is irritating. For those unaware, from Valak Mountain onwards (with a couple area exceptions), enemies can drop the advanced art manuals. The catch is that the manuals only appear in Gold Chests, which is about a 3% chance initially (Riki has a skill that can increase it but no clue by how much). And then Gold Chests still only have like a 5% chance for its last two item slots to have a Manual... which will be one of four at variable success rates.
There IS a way to make this significantly easier, mind you - Unique Monsters
always drop Gold Chests. And yes, starting at Valak, that includes potential manuals.
There's a list here. You just save near the monster, load in, kill its face, and hope it drops a manual - and if not, reload and try again. It's still a mild pain in the neck though... @_@
Edit: Nearing the finale of the game. I've got like two areas to go:
. The game finally got around to answering its plot threads and holy
shit did it ever answer them @_@.