Those Spider Warriors (or whatever their exact name) are basically heavy infantry with the stealth ability. They are, according to their description, assassins-in-training, but they have not yet graduated
What you can do with them, besides using them as (rather expensive, but powerful) heavy infantry in your normal armies, is put a few (say, 20) of them under a stealthy leader, go behind enemy lines, then take a lightly defended province (attacking it in the usual way). Then sneak back out when the previous owner comes back, with his army, to evict you, and go take another. While you are at it, you can put taxes to 200%, pillage, destroy buildings, in short make a perfect nuisance of yourself
Cutting off the retreat options of the enemy's main army just as you engage it is also an interesting possibility...