Theres many wheres and hows to this. Many guides online. My personal opinion is, and what i did myself. Was to start focusing other aspects of the grind. Like specific drops for enhancments or treasures, or marni stones, and let the XP come to you as you do that. If youre looking for just straight exp. You cant go wrong Elvia Orcs, Saunils, or Nagas/Biraghi even. Whatever youre most comfortable with and you clear quickly. Also always best to be on the look out for XP Hot Time Events to ease the pain of the grind.

IF you have the gear for Gyfin Underground, I believe that is still the best xp spot in the game

Apparently the night time +exp buff is a multiplier, not additive

So if you have a limited supply of an exp scroll, save it for night time to get more mileage out of it

That said, levels past 62 arent particularly rewarding

I think most people would tell you to focus on other goals and just let exp be a bonus

Or if level is semi important to you, focus on other things until hot time event and then grind yourself silly 😬

Gyfin upper groups are always available in Part finder (Search Party in new UI) , Very good exp and easier than underground. If you have geared friends they can carry you through some hours to level very quickly.