Did you disable the two generators to take down the shield? IIRC I was level 23 when I did that area, and didn't have any problem.
There is a third cave in there, as well, which you need to clear before defeating Barnabus, or the last room will remain locked (there are 3 teleporter pads in the cave, one at ground level, one on a ledge mid height and one on a high ledge).
The easiest thing to do would be to explore a different area first (the beach and depleted ore mine), and come back at a higher level. It is possible to take out opponents 5 levels above you, but if you are not doing any damage at all, then returning later would probably be easier than trying to find an effective tactic.
The Camp Sentinel View teleporter will bring you to Camp Eagle's Nest, which is on the west side of the island in the middle of the channel leading to the harbour. It is also possible to fly above the anti-dragon shield and drop down in human form, if you find the right spot.