Once you get down to the water, there is a boat there you can jump onto. If you stand at the right spot you can target opponents on the far beach with a bow/magic, but many of their attacks can get blocked by the back of the boat. I noticed that by accident, and stayed there until I had thinned out that end of the beach, and it was safe to swim over (a creature with a mage/ranger head can make a decent shield, as well).
Alternately, you could swim/fly around to the other side of the beach, and try from that direction.
In the sinister cave, there are 3 teleporters (one high on a ledge, one mid level on a ledge and one on the ground) you need to go through and explore the areas they bring you to (two of which have generators that need to be disabled) before taking out Barnabus in the centre of the cavern.
If you fly out over the water from the start of the fjords, you should see a bunker high in the mountains behind some anti-dragon shields and defenses. That is where you need to get to lower the barrier and disable the shields (there are a couple traders in there, as well, after they surrender).
You can either use a teleporter to get up there (the Camp Sentinel View teleporter, on the west side of the island in the middle of the channel, will bring you to Camp Eagle's Nest, where you can access the bunker) or it is also possible to fly above the anti-dragon shield and drop down in human form, if you find the right spot. The teleporter by the blind philosopher will be activated once you disable the barrier.
