The bunker is high in the mountains south of the grand knight teleporter shrine, built into the cliff face. From the teleporter, turn into a dragon and fly across the water heading for the south western end of the island in the middle of the channel. The anti-dragon defenses will start to attack, but if you keep moving to the island, and transform over a ledge there with a teleporer on it, they shouldn't be a problem. The teleporter there is how you get to the bunker behind the anti-dragon shields, in order to disable the defenses and barrier.
There are a couple traders upstairs, after you've secured the lower floor, where you may be able to get better equipment, etc, and sell off loot you don't need.
You can access the Dragon Cliff Castle, on the northern side of the channel, after lowering the barrier.
There is a cave near the start of the fjords, high above the waterfall cave (follow the water up and you'll be in the general vicinity).
Return to the teleporter on sentinel island, use your dragon stone to teleport to the throne room, and send your runners for malachite ore and diamonds to sell. Wait until they leave, then exit and return; once fully upgraded they should have a pretty good success rate, and you can do this repeatedly to get potion ingredients or alchemy ingredients to use or sell.
What type of character are you playing, with what skills? At level 20 you shouldn't be having that much trouble with the level 21 imps, and the Black Ring members on the beach under the bunker should be easy.
