Co-sign accumulate/focus, lets you get a good amount of JP in those first few rounds when you're not doing something, not to mention how much insane jp you can get if you leave one enemy on the field and just accumulate up until you run out of medical items from that enemy hitting you. I always feel that's the point at which I've gained enough JP and it's time to see how my characters are developing and to make adjustments.

I agree with equipping JP up all the time, it does take away from a big part of the game and deprives you of tactical options which could really help in some story battles. I like to keep it on my characters for optional battles, as well as the Squire commands.

I'm not sure if this is true or not, but for some reason I thought the fraction of JP characters get in the jobs of other members of your party when they take actions only happens on close proximity. I could be wrong, though.

It may sound obvious, but I always level up characters in a job to the max of level 8 before changing their job to one unlocked by the previous one. It makes them much more effective in that later job IMO.

Hard for me to say what the hardest fight in the game is. The whole second half kinda just blurs together for me as a series of challenging battles where i'm spamming my cheap and overpowered moves before the enemy can do the same.

But I would recommend having multiple saves, especially when heading into a fortress. The game borrows from its predecessor Tactics Ogre in forcing you to complete a series of battles in one area and you can't back out. Unlike Tactics Ogre I don't think the game did a very good job of letting you know what the stakes are. Again, could be wrong on that.