You can always return to Broken Valley and finish off anything you have left to do there. In the far south west you can take the path to the Orobas Fjords, to level there. That is what I did before heading to sentinel island; the first couple level 18 opponents were tough on level 15, but they brought be up to level 16, and that made things easier. I had my creature using a warrior head, and after awhile (when I ran into trouble against some level 21 imps trying to get another 4k experience to reach level 18) tried a ranger head, and found him much more helpful.