I was almost to level 16 when I reached sentinel island, so rather than waste the first mindread on the island character to go up a level when I only needed 3k experience, I went to the Orobas Fjords to level. The first couple opponents brought me up to level 16, and as long as I was there I went down the path, reaching level 17 at the bottom (level 18 opponents). Crossing the water by the falls, and then swimming over to another beach (level 18-20 opponents) almost brought me up another level. I had to take out 4 or 5 level 21 imps to get the last ~4k experience to level again, which took several attempts, even using the creature as a distraction.
You may want to clear the first section of the mountain path in the fjords until you get to the teleporter shrine, but even that is optional before you have the battle tower.

If the high level characters decided to take you on directly when they first wanted you dead, it would be a pretty short game. While it probably could have been done better, the antagonist leaving the protagonist for dead with an elaborate scheme and/or peons to finish off the job is a time honoured tradition in games, movies, etc.

The killer bunny can not climb, so you should do better if you take out the last rabbit near the bottom of a steep riverbank or close to a large rock you can jump onto (Killer Rabbit).