The game isn't easy even on Casual mode and since it half goes back to the old level grinding ways here are some tips to help you through the game where you'll most likely struggle.
JP Grinding, Max Out Jobs Quick
Halycion, right from the start of the game you can go to the NE of the city where you can find wasp-like Vesps who will give you around 50 JP per battle on their own. However, they also drop nectar which means you get a random number of consecutive battles and sometimes 400+ JP.
If you have everyone main as Freelancer so you can max out their level to 12 for the "JP Up" abilities and sub job as Black Mage, you can make quick work of them. Of course, bring some tents, ethers to recover quickly and use Lunar/Solar to reduce downtime.
So you basically have everything you need in one place. Took about 2 hours to max Freelancer.
By Chapter 2 you can fight the Paraponera ants next to Wiswald which also work the same way by dropping nectar for you to have consecutive fights. This is fairly much good for the rest of the game for levelling jobs as the ants as there's little downtime in between fights.
You might want to have Gambler unlocked via a side quest in Savalon too during chapter 1 while doing this to gain the "More Money" multiplier. Just speak to Shirley in the casino place to play a mini game of B & D. It's kind of like Reversi or The Rose King game.
Growth Eggs, 2x JP/XP Accessories
Once you beat the game once, you hit chapter 6 at which point you'll be able to do two side quests for Glow Egg accessories to get an XP/JP x2 multiplier. What's more these multipliers affect the XP/JP items you use too such as those from setting sail unlike the job abilities which only affect battles.
Right from the start you can get 2 Growth Eggs with Quest 92, "Doctor's Orders" which you can get from the Serpent's Grotto just north of snowy Rimedhal where you met the dragon. You'll then have to make your way through the Forest for a boss fight with the undead Xolotl to collect some Calmagrass. My team was around level 58 when I did this fight.
You won't be able to get two more until you've defeated Night's Nexus.
Once you've "defeated" The Night's Nexus once, you can go find the other 2 eggs with Quest 100, "The Collector". South west of Holograd there's a Magma Mountain where Ignis Elements seem to be the best place to farm the Oberon Gem drops even though the quest pointer leads you elsewhere.
You don't just get drops, you can also steal 2x gems from them. Of course, you'll want to equip the "Rare Talent" ability on everyone which you learn at Gambler level 10 and will increase rare drop chances by 10% by each member equipped. Having Soul Food to attract multiple Element battles will help speed things up too of course.
You might want to equip the Freelancer's Dungeon Master ability as there are some jets of steam that can damage your party.
Enjoy the two credit rolls? Think they overdid it this time but anyway... Now it's time to prepare for the true ending.
You'll also have to unlock the other 3 hidden jobs to continue with the game; Gambler, Salve-Maker, Bravebearer. Salve-Maker can be obtained by completing sub quest 47 "In Dreams" in a small snowy village of Enderno just south of Rimdhal.
The Bravely job can be found in the Vale of Sighs, far east of Halcyonia at Sloan's resting place. If you don't remember where it is, enter from the Halcyonia side and don't use the teleporter. Ignore the first slope and follow the path deeper into the vale. Once you reach the second save point, head up the middle path to find the spot and be prepared for a boss fight.
Bravebearer Sloan Boss Fight
It's a tough fight so be sure to save as Sloan hits very hard with both physical and magical attacks.
My setup isn't the fastest as I've seen level 30s win the fight in around 2 minutes but since they didn't go into detail about what abilities and gear they had, I ended up doing the following instead.
The key is to have Phantom mastered as the main job so that you unlock the second specialty "Results Guaranteed". Then you can use Ranger as a sub-job so you can use Shadowbind to paralyse Sloan. Phantom's second specialty will guarantee the ailment effect or it won't land. Shieldmaster will have Vanguard subbed to keep the Phantom party member's BP topped up so that they can keep Sloan paralysed. Everyone else can just focus on attacking and he shouldn't be able to even land an attack.
Onto Chapter 7
Now the next optional step is to go gather the best gear you can get in the game and also unlock the job level caps along the way to help make the final fight easier. Learning the Bravebearer's "Obliterate" ability will make job levelling even easier as it's instant death for enemies 20 levels below you; pretty much the Yokai ability from Second. Wiswald still remains the best place to level at the point IMO.
Once you have these remaining jobs unlocked, you must save the game but not the usual way. Instead, you need to save in the Night Nexus slot that appears at the very bottom to progress onto chapter 7.
The final story boss isn't actually that hard. I ended up winning without passing the tribulations to get the ultimate gear at low 70s. The tribulations themselves are actually much harder as you're up against all the previous bosses in teams.
Ultimate Gear & Bonus Boss
I'd leave the very first tribunal that's south west of Halycion last because it's the toughest. You can re-fight each tribunal to hopefully get ultimate weapons drops but the rates are terrible even with a full party equipped with Gambler's Rare Talent ability. You could also try your luck at stealing some armour from them.
I ended up level grinding the remaining levels in Fount of Knowledge via the fairy realm using Obliterate since I had plenty of nectar left from levelling in Wiswald. Once again you go after the ants. Sometimes you can come across the Queen Wiki-Wiki which can really boost your rewards.
You end up learning so many skills at once you'll be overwhelmed at the results screen but just keep hitting the B button and you can skip the notices.
Only tribunals I had problems were 6 and 1.
For tribunal 6, the Berserker and Arcanist can hit very hard without some debuffs. I ended up defeating the arcanist first while debuffing the Berserker's physical attacks. Then finished off the Pictomancer before the Berserker. You also need Safety Rings or Circlets to prevent instant death from the Pictomancer.
Then for tribunal 1 you have Sloan teamed up with two other bosses and he is now immune to paralysis which made it even harder coupled with his OP attacks.
I ended up taking out the Black Mage first then continued to keep Sloan debuffed, Reraise up using White Mage's second specialty so everyone gets it before defeating Sloan then the Bastion. Only downside was Sloan was always revived if you did it this way but I managed to win in the end somehow.
If you manage to beat all 7 you can head up the stairs in Rimedhal where you can reach the graveyard and fight a bonus boss as with SE JRPG tradition.
Here's my so elegant "zombie monk" tactic in action.
You don't get anything from the boss. Not even an extra cutscene unlike Octopath that rewarded you an item to let you explore the world without random encounters at least.