Have you completed everything in Broken Valley? (assuming you don't have the battle tower yet)
After finishing Maxos temple, I headed to the fjords to level before going to Sentinel Island (you can mindread your first encounter there to go up a level, so the higher level you are, the more of a boost that will give). I was level 16 after the first couple opponents, got to 17 at the bottom of the mountain path, and needed to take out a few of the imps to get to level 18. I found it much easier after switching my creature to a ranger head (level 2 Fireball) rather than a warrior head (level 6 Rush Attack / Fatality), but it was still challenging on level 17. I later switched my creature to a mage head.
There is an experience bonus for taking out higher level opponents, but it is capped; at 3 or more levels higher you get a 50% bonus (1/6 if they are one level higher, 1/3 if 2 levels higher). Generally, if you start running into really difficult opponents there are other quests or locations better suited to your level.
Welcome to the forum.
