[Baalz]

Every SC has holes in its defense. Usually, by their nature SCs are invincible vs regular troops, but the right spells can take down any guy.

If he's undead, look to spells like solar rays, dust to dust, etc.

Leverage whatever elemental resistance he's lacking. Thunder strike/orb lightning or frozen heart or incinerate.

Another option is to fatigue him out. That impressive regen doesn't help him recover from ghost grip, stellar cascades, or the heat aura from fire elementals.

Finally, just equipping a few reasonably strong guys with good weapons will usually work. If you don't have any good candidates on your recruitable roster, think sleepers or bane lords. Theres some real good anti undead/demon weapons if that's applicable, moon blades are pretty fierce against magic beings, smashers work well against constructs, etc. If none of that is applicable you can go with something like dusk daggers/gate cleavers - his regen isn't gonna matter much if you've got 4 or 5 quickened guys hacking at him with those. You can also go with axes of hate which will fatigue him out, or flesh eater axes to give him a chest wound (which is pretty terrible if he's not 0 encumbrance)

Also, don't forget to curse the bad boy as soon as he starts looking for trouble.

Finally, there are some high end spells which work regardless of his defenses. Life for a life, claws of cocytus/inferno, and leech in blood. Petrify, drain life....probably a couple others I'm not thinking of right now. Gifts from heaven (with some precision boosts for the caster) is also pretty smushifying, as is banefire.