[Kobal2]

Fatigue is the SC killer.
Fatigue is the little death that brings total obliteration.

Stellar Cascades, Ghost Grip, Numbness, Petrify, Paralyze (with high S and pen. boosters, it's easier to cast than Soul Slay so an S4 caster will have +1 pen bonus on top of their gear), Sleep spam, summons with heat/cold auras or in a pinch and if the SC is not fatigue-neutral, a massive skellyspam.

That's what I'd try first.

If that SC relies on self-buffing and doesn't have a tower shield, a handful of air or nature mages self-buffing eagle eyes/aim then letting loose with bows of botulf will neuter him. Even better if you can Quicken them.

If you have the blood/astral combo, repeatedly sending horrors at the guy will work. Eventually. And in the meantime, if you can send a lot of them at once then you stand a chance of stacking enough fear auras to make him run and cancel his orders for the turn (or even kill him if you've cut off his escape routes). This of course assumes the guy is on his own, if he's in the middle of an army this particular trick won't cut it.