[Tiavals]

Use flying troops that attack him before he can buff himself(provided he doesn't have bodyguards). That way he must melee and can't cast.

Also, using thugs is good too, if he's geared against normal dudes. Mistform and etherealness are useless against magic weapons, and armor is useless against armor negating and piercing weapons. Get multiple attacks if possible to lower defense, or attack upgrade items.

As for spells, immobilization and such spells work well with thugs, since it lowers defence, which is the biggest problem for thugs against SCs.

If you feel lucky, you can use those insta-kill crossbows and armor negating crossbows and so on.

All in all, magic items is the way to go, I think.

Then again, I've never truly faced any SCs in a game, this is just extrapolation. :P