Grinding is not a great aspect to have in any game but in games like this one it is eventually going to happen since you have 8 characters to choose from and you can only have 4 of them in your party at any given time.
In other games I have played the people that are not in your group would also gain passive XP - not 100% of it but some - and I am a bit disappointed that this is not a feature in this game. If you park one of your less favorite characters, which for me has been "Agnea", the dancer, she will remain at the same level that I last used her at until I choose to put her back in my party.
I just don't care for her story as much as the others and I also don't find her special attacks to be terribly useful. Other people have countered this idea and said I am wrong but when it comes to gaming, no matter what your opinion is it is going to be wrong in the eyes of someone.
Anyway, when it came time to do her 2nd and 3rd quests, which I presume is necessary for defeating the game, she was more than 20 levels lower than almost everyone else in my group. I just didn't feel like using her.
Since we have done everything else that can be done as far as I am aware barring me simply not realizing that certain places exist, I had no choice but to shove her into a party and then head out for random battles. The obvious thing would be to put here in the group and then head to a high level area where she is going to get massive XP with any kills that we do but here lies another problem. If anything gets a chance to attack her during that battle, it is going to one-hit kill her since she is also 20 levels below all the enemies in that area.
So what you can do is to just take that chance and resurrect her every time she gets downed or you can go to low level areas first, which takes longer but it easier to keep her alive in.
I have discovered that there are some items that you can equip that will make her and everyone else's XP gain a bit faster though, and they are hidden in plain sight because you probably didn't pay much attention to them when you were awarded them.
You can add Osvald to your group of if you have leveled up any other character's job skills as the "scholar" enough you can equip the job skill of "Extra Experience". This will boost all XP gains with an additional 50%
You can also put Throné in your group and equip a skill called "Life in the Shadows"
While it doesn't specifically tell you what the bonus is, this ability or skill boosts XP and JP gain after every battle, but only at night. The day/night specification is kind of irrelevant since you can switch to day or night any time you feel like it by pressing a button. For me, on the Switch, it is the back R button. Just keep everything at night all the time because Osvald's boost works around the clock.
There are special enemies called "Kates" and "Octopuffs" that when killed give a lot more XP whan other regular enemies and you have some items you can equip that will make these enemies spawn more frequently.
I'm sure this is great and fun for the hardcore people out there but these enemies also flee from battle if you don't kill them very quickly so at least in my situation it was too much of a hassle to focus on trying to encounter them only to have them run away before you can do enough damage to "break" them into staying until they die.
No matter how you decide to play this game you WILL encounter a situation where you are going to have to grind for XP simply because there are more characters than there are playable areas. It seems like a real drag until you equip these items I mentioned, at that point it will all happen relatively quickly. It's still kind of lame that you have to do this though. I would say that it would be a good idea.
You can also equip the Merchant skill of "Grows on Trees" to boost the amount of leafs (the name for the money in the game) that you get per battle and this is always helpful since a lot of the late game items are quite expensive, especially when attempting to acquire a particular vehicle that I am
not going to mention for the sake of not spoiling the game.
There isn't a ton of necessary grinding in this game, but there is some. Thankfully with these tricks you wont have to feel as though the game is a drag because you have to do it.
Have fun!