Well, you can try putting him into a position where he either can't summon more spiders or they can attack your chars in a way you want.
I remember beating the spider on impossible with giants, shamans, archmagi, inquisitors... Don't remember the last one, but it was ranged too. Shamans put totems all around the army. Then you try to diminish the enemy troops around the totems to like 1-15 units (and you don't use earthquake) only leaving your giants to tank with shield from the archmagi. That will make the small troops of spiders to attack your totems (careful with the totems that attack, make sure you put them around the giatnt and the healing totems around the weaker troops).
It will take quite a bunch of rounds, but you will do it. I managed to do it with only a few archmages dead (because they happened to be close to the giant spider and he started attacking them). You will manage as well
Another thing to do is to try and get some royal thorns. Spawn as much thirns as you can and this battle is a breeze.