If you are going solo I would recommend these zones.
20 - 22
Nektulos forest right at the entrance from the commonlands there are cackling cadavers, bats, and spiders that you can grind on till 22.
22 - 25
Once again in nektulos i would suggest fighting the owlbears and grinding on them till level 25. (You can also get the Shiny Brass Halberd HQ and complete the owlbear part of the quest)
25 - 27
Also in Nektulos, I go to Bone Lake (which is at the top of Nektulos Forest) and fight the fairies right around the lakes edge.
27 - 29
This is also in Nektulos Forest near Nektropos Castle. This spot is in the valley before nektropos castle, and you can fight cackling cadavers, patchworkmen and gargoyles.
29 - 30
In Enchanted Lands right near the docks there are turtles that you can grind on till 30.