At the start of the fjords there are a couple caves you can get to in dragon form. If you clear down the mountain path, cross over to the opposite beach, then swim / fly around the corner of that mountain, there is a path that goes past the depleted ore mine and leads to High Hall and the harbour (the path eventually splits).
If you've done all that, and explored around the fjords, the next area to go to for higher level opponents would be Stone's Flying fortress, with a teleporter entrance on the mountain island in the middle of the channel leading to the harbour, on the north east side.
Technically you get more experience if you mindread first, where possible, then focus on combat and then turn in quests. Ultimately, though, you are still going to end up at pretty much the same level if you do everything.
If you don't have your dragon form yet, you'll need to return to Maxos temple and take the teleporter there that was made available after Amdusias was defeated.
Your first encounter after you use that teleporter you can mindread to go up a level, so the higher level you are (I made it to 22 in the fjords) the more of an experience bonus that will be. You don't need to get to a high level, but it would be better if you got to a new level before entering, to get a full level bonus (equip anything you have with a Wisdom boost). If you have done everything that you can, you might consider reloading a save just after going up your last level (if possible).
