my party is at lvl 20 and the enemies in the mines are usually under level 10...its a small area too.the swamp is pretty good for me. there are no difficult enemies there (other than the occasional high level horse or rynjin), there are lots of them, they spawn often, and most are quick kills...like the bugs. all give standard exp (usually 12-60k).
ive been to every "zone" other than the last 2 easter eggs, the wilderness clearing, the rapax areas and ascension peak...the swamp has been the best for *me* *so far*.
if you want to build up anywhere, turn on x-ray, hunt every last critter down and fight them like it was the end of the world (use full buffs, and all kinds of magic). then take a nap in the corner, maybe teleport to and from a town to sell the loot, and when you come back, there should be more spawns. this way you're constantly fighting, selling your gear, and increasing your magic skills. obviously if you need to increase non-magic skills, use lvl 1 magic attacks. they do squat damage, but you still gain skill.
I recommend the huts in Bayjin.The big advantage is, that you can just close the door and rest in peace 
The battles are long but not very difficult since you can fight your enemies one by one on the doorstep. The encounters mostly feature rock / curare crabs and ryjinn sentries and the last battles i fought there yielded 100k+ each.
Of course, Saxx and Vi won't follow you and you have to clear the village first, but otherwise a great place.
duck