Well, they got to the chasm, and wow ...
They defeated the first set of sentries, who heard them and were ready for them.
They then went down the tunnel, held a light above the hole, and discuseed tactics, then lowered the invisible barbarian down on a rope (the rope wasn't invisible, and the spider's tremorsense gave them 6-8 rounds to prepare. But hey, they defeated them too - but one sentry, whilst being pelted with arrows, got away and climbed down.
Then they cast daylight on a stone, and carried it to the edge of the chasm. Didn't that get the attention of the spider in it's nest 60 ft away...
So, we finished the session last night on round 10 of the combat, the spider just fell into the chasm, dead, and the web team is coming up to meet them (50 ft away), they're pretty much out of spells, beat up, and the barbarian is now fatigued...