Goldeneye2013 and QuintusV have created an XP farm using 4 separate spider spawners. This makes the farm super effective. When you stand on the spot above the red concrete all 4 spawners will be active. It won't take long until there are a lot of spiders to kill!
You can find the XP farm at -1180,63,-120
Up the ladder is a spot to kill cave spiders. Don't stand too close or they will poison you! To get the XP and drops from the cave spiders you will need to go down the ladder again to grab them.
There is an e-chest available and you have acces to the dropped string and spider eyes.