The BFs are randomly generated the first time you enter in each act, including the quest items and monsters. In general, though, spiders have a very high resistance to poison and a very low resistance to ethereal damage. They have moderate resistances to water and earth damage, but are a little vulnerable to fire and air damage. The best conventional damage type is piercing, but there is not a huge difference between their resistances to physical attacks.
If one character is an archer, or mage, you can have them do most or all of the damage from a distance, while the other character hit and runs to provide a distraction. I kept my warrior in the lead while exploring, and set my archer to aggressive (right click on character portrait) to automatically attack opponents in range.
You could also just return when you have leveled up a bit, or have gotten better equipment.
