You can have your characters sneak, then approach, use a ranged attack and retreat (so the spider doesn't get close enough to see you and counter-attack), then repeat. If the area is large enough or you can attack and retreat in a circular path you shouldn't have any problems.
You can have one character use a bow/crossbow or magic (though the more powerful spells are not available until act 2) while the other character either runs around distracting an opponent or uses hit and run tactics, allowing the first to do most of the damage from a distance.
If you are not getting many hits or keep getting interrupted, you could try using potions to boost your speed and survival. An agility boost would help your accuracy, and depending on the weapons you are using, a strength boost would increase the damage you can do. Alternately, you can just visit the potion dealer and stock up on healing potions.
The math appendix in the
Beyond Divinity Strategy Guide shows the secondary effect of the primary stats, and has recommendations on stat point distribution (though even with an ideal character build, an opponent time and a half your level you be at least a bit difficult).
