I managed to farm this boss for almost an entire level with minimal casualties. When I killed the boss, it was dropping stacks of almost 600 spiders.
I had about 8,500 leadership so I had within 10% of the max:
Giants
Shamans
Alchemists
Cannoneers
Archmages
First, I started my units in a position where they could all easily fit in a corner. I put my giant up front by the spider (until he moved) to take damage instead of spawn spiders on some turns, and I could heal the troop every turn. Immediately, I had my shaman troop build a wall of totems around my army. 7 stomping giants in conjunction with multiple death totems proved very effective against the mass spawns of spiders. I dedicated the Alchemist and Archmage troops to helping pick off spiders. My giants stomped every other turn, and hit the boss every other turn (while it was on the correct side). My shamans were constantly building totems, I even used "gift" a couple times to speed it up. My cannoneers were the majority of damage to the boss, using their triple cannonball attack. The rest of the time they helped pick off mobs.
By the time the boss was down within 1k HP the spiders were up to 550 stacks and were starting to tear down my totems. I didn't lose any units until the final turn, when 2 stacks got to my alchemists/cannoneers.