Ah Rune i think after my bout as a Ninja i can enlighten you quite well. For the first 4 levels in quifim we fought worms. It started with a ranger, thief, and myself. I highly recommend getting a ninja who is not cheap. What happens is the ninja provokes the worm and stays out of melee range. The worm will eat up shadows otherwise. The only spells that will get through are stonega at Quifim levels and sadly silence. It wears off pretty quickly though and so should not be a problem. So i had two parties doing this and this was how they looked.
Rng, Nin, Thf, Thf, Smn, War
Second was this :
Rng, Rng, Nin, Smn, Smn, Drk
As you can guess, that second party was crazy. Sure rangers can solo, but this route was much much faster. I really think you need a ninja though as stone and stone 2 as well as rasp and bind will all be absorbed from shadows. I see no reason why this would not work later on as well. I realize they will have stronger AOE magic, but they rarely use it more than once per battle. Though i have seen 4 times in a row >.<. Go for it, was probably the best exp ive ever seen.