You can head to the fjords early, if you wish (take the path to the far south west in Broken Valley). Only the starting section is available before you take over the battle tower, but there is lots of experience to be had there.
You may be past this point now, but your first encounter on Sentinel Island 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 is better to reach a new level before entering, to get a full level bonus (and equip anything you have with a Wisdom boost, so rescuing Rothman first would be better).
In Laiken's study, did you have your creature summoned (and possibly a ghost or demon, if you learned the ghost skill or have jewellery with a bonus to either), maybe lower the difficulty setting, use stat/resistance potions, etc? Depending on your character build, a change in tactics may help. I think the abominations there are level 17 and 18, so you shouldn't have to get too creative to survive. Level 1 of Evade helps in close quarters, even if you are playing a ranger or mage (higher levels of the skill don't give as much of an effect per skill point). Did you check the merchants in Broken Valley again since you last levelled, for equipment or enchantment upgrades?
Speaking of maybe missing things, did you do any swimming around the island? There isn't much to find, except near the windy peninsula.
Jump into the water to either side of that peninsula, and you can swim to the entrance of a hidden cave.
