[thejeff]

One thing to keep in mind is that a lot of advice is geared towards MP, because that's what most of the experienced posters play. Small numbers of elite units is the efficient way to go, so that's what the standard strategies counter.
SP is easier to beat, because the AI only has only has one real strategy and doesn't try to counter yours. You have to figure out how to effectively kill large numbers of troops.
Cheap raiding thugs work in MP, because no one invests in PD or has large armies wandering around in their backfield. And there's no point investing more gems into them, since if they run into a real army with mages, they'll die anyway unless they're a full-fledged SC. Or they'll get hit by something designed specifically to take them out.
In SP, you don't have that problem, but you'll generally be hitting stronger PD and larger armies so the thugs will need to be tougher. Which is OK since they won't be targeted by anti-SC forces.
You were playing C'tis? Have you tried Banelords? (Or Banes for a lesser verion) Maybe, Fire/Frost Brand, Vine Shield, Horror Helm, Luck Pendant, Antimagic Ammy or regen if it works on them. Flying shoes if you can get them. The default armor is good protection, Forge something if you need specific resistance or a special ability.
Wraithlords, with similar gear and casting Soul Vortex are better, but more expensive.