When I got to him, I had considerably less troops than I should have had, and no ranged troop aside from the inquisitor. So I had my griffins and horsemen take care of the early spiders while my shaman dropped totems like mad all over the battlefield, to fill up spaces
. I just had acces to a demon portal, so I used that 4-5 times, also 3 phoenix scrolls, summoned demon's summon demon spell (heh, makes no sense) and eventually I filled up all the battlefield save from a few controlled spaces, with the demons and summons to soak up the damage. All the while, my inquisitor and my royal snakes (teleported around when the spider changed places) dealt damage to the giant spider. Source of magic to keep up my mana, Greasy mist on the spider and hell's breath on my snakes, when mana permits (the demon portals are expensive...).
Nevertheless, I did the fight with no losses, but it took virtually ages...