Wait till the spider walks onto the web so you can target it at range. Attack from stealth -> Surprise round. With a party consisting of 1x mage, 1x hunter, 1x cleric, 1x warlock. Round 1:
• Mage uses upcasted magic missiles with the amulet, average damage of 28.
• Warlock uses Hex+Eldritch Blast, average damage of ~13 after taking into account hit chance.
• Hunter uses Hunter's Mark+attack, average damage of ~15.
• Cleric uses Guided Bolt, average damage of ~14.
During the surprise round you have dealt ~70 damage to the spider boss. If 1 person, lets say the warlock, had targeted the web instead, they would do 48 damage instead of 13 damage, taking the total up to 105. Either way, provided nobody dies, the spider dies in 2 rounds. Environmental gimmicks not required.
Thanks for making me realize that I'm a retard for having a Fighter/Cleric/Warlock/Thief main party.