I haven't beaten it yet because I need higher level first but I have an idea. With Soothsayers you can charm a spider unit(unless it's undead) and it will create chaos among the other spiders.
If you were lucky and got a Colossus call scroll you can use it to summon a lvl 5 troop, which varies depending on what island you cast it on. But first make the crow copy it so you can make more. I managed to get a Black Dragon with it. Otherwise there are some really nice tips here. I think I will use Royal Snakes myself.
Btw, Is there any way to get a map to another continent before beating the spider boss? Would really love to go somewhere else and level for awhile(also, to get free goodies, hehe). So far I have the first four islands.