From the entrance of Mandrake be invisible and go two southeast, two south, southeast, two south, southeast, two south, four down, twelve south, twelve west and four south. The mobs in the cave can detect invisibility, but the ones outside can't, so invisibility makes it easier to just bypass them. The caves are npk, though, so keep an eye on where and who forest.
Glamdrung is one south, and aggros, but isn't too hard for melee classes like barbarian and cavalier, especially so for valkyrie. Slash damage does not harm him, so use pierce, bash, or exotic. He doesn't cast spells, and drops nothing of importance except for the key to the down below, which is cpk and has an archon mob so don't go down.