[Kobal2]

As Fantomen says, a sneaky way to mess with a skellispammer who's only using a bunch of mages and little to no supporting army is to script spells that won't affect mindless or undead units. Cold spells would work too, since undead are immune to them and the mage casting them would be forced to target the spammers, although they probably won't have the range or precision required for the job. But things like Mind Hunt, the Northern Star/Magic Duel combo, Harm, Sailor's Death, Stream of Life... could all become your Sauromancer sniper rifles. Heck, even lowly Sleep could shut them down quick.

I've had great success with Air elementals, too - they fly, so they get to the spammers very fast even in a siege. Usually they'll target the undead first (more's the pity) but occasionally you'll get lucky and they'll roll over a sauromancer or two.