Pollys Forest is by far the best intermediate grind-zone. The xp here is super super high espically when combined with marni stones. With that being said, this spot is great you can grind Polly's for tons of skill points and xp. Pollys is the best grind spot in the game for skillpoints and the second best spot for xp right behind Mirumock.
Pollys Forest is located West of Grana; Grana is located South of Calpheon the third major city in Black Desert.