The best concentration of spiders in Erud Luin is in Talath Ondren, which is the eastern end of Haudh Lin, near the river. This nest also has a chest with the 5th gear for the gear quests from Bavor in Thorin's Hall.
There is also a short quest in Talath Ondren to rescue a stranded person hiding in the hills from the spiders. So doing that quest will give you a dozen spider kills at least. If you search the hills by the nest, you will see this NPC.