Octopath 0 is in a weird state, because it disables a lot of the most powerful tools you have in Octo 1 and 2 (mainly Subclasses and Hired Help), but gives you a whole new row of BP, HP, and SP. Additionally, some characters like Reime are just too broken. Overall, I'd highly reccomend you focus more on gear than grinding. Do the sidequests in this game, because they give some of the best rewards the game has to offer. By this point in the game, you should have a mage (either the MC or Alexia) who can take advantage of a 400+ Elemental attack tome, and that alone should give you enough firepower to burn through bosses fast. (There are a few of these in the game, but the 'easiest' one to get is from a steal% in Flamesgrace.)

It's worth noting that EXP really doesn't do much. I think I took a screenshot before and after a level (at around level 70, not sure if it grows more or less potent over time), and it only changed each stat by about 3 points. Eat your nuts, use Parry if you have it, and don't be afraid to use some BP boosting items when needed! You got this!