There's a couple good ways to farm the xp to get to level 100. I suggest you wait to farm until after chapter 4 or 5 and unlock the sidemission that gives you an aspect to earn xp even if that charecter isn't in combat.
Anyway the first good way to farm is using the grimore and doing egg hunt sigils. The first sigil might be a pain to get but they are easy to farm after. Once you get your first egg hunt sigil save and go into it with Rinna on your team and her Steal dive force equipped. Each floor can have 3 eggs so keep reloading the grimore until you find a sigil egg. Switch to Rinna and set everyone to not attack. You can then use steal dive on the sigil egg to steal more egg hunts. Once you steal one run away from the fight and the enemy will still be there. You can keep stealing and then running away to get more egg hunt sigils. After you have a few extra keep running the egg hunts killing the eggs you find. There are white eggs called XP or million eggs that when defeated give you thousands of xp at a time for 5 seconds of combat.
A second way that does burn through money is at end game as long as you can afford it buy master egg sigils along with a dragonshift zest sigil and itll give a 40K xp as well for a minute or 2 of combat. Master egg sigils don't stack but a master egg sigil and a xp sigil will stack to give you almost 60k xp a fight
The third way is after finishing the game go to Glaucus falls. There's a level 100 enemy that spawns that gives about 4000 xp every time you kill it. It only takes about a minute to kill and you can keep leaving the area and coming back to get it to respawn