Efficient use of SC against richer nation

[Taqwus]

vs. the AI? Wreck their temples (the AI does not necessarily protect temples very well), raise taxes to 200% in your path, interrupt supply lines if their armies are venturing far from their forts. Sieging is slow with SCs unless they're very, very physically strong or have siege-bonus equipment. That is, if you're going to use them separately.

Using them to lead your armies to destroy the invaders would probably be a better tactic, unless your mages and regular forces can do the job.