I still havent try this game, but in previous KB spider had same characteristics : it`s head attack were poison and did lots of dmg to all non poison resistant units, but it you attack it from sides left/right legs retaliation is physical so get some high def troops on it`s sides cast stone skin/magic spring if available and start hacking.
As windezz said try that hut for trolls or jotuns, but if fight is underground troll is better choice with his regeneration, and i`m not sure if this still stands but every retaliation is in 2 hex area so dont attack with 4 units at same time, because when you hit 1 spider leg it retaliate on 2 hex dmging 2 of your units (to much losses for your no revive unit setup). Good luck
