Like iRFNA said, some combination of magic paths and research can sink any SC. Otherwise, the larger the army the more you need to cut down the number of hits your SC has to deal with per turn. A Shield of Gleaming Gold and a Horror Helmet are good for fighting troops with normal to low morale. A Vine Shield is the best for weak troops, especially in a province with Growth scales.
But, taking down a 500 unit army is too much for just about any SC regardless of how indestructible it is due to the limit to the number of turns for each side in a battle. I believe that's 50 turns for the attacker.
Baalz has an excellent guide for thugging here:
http://forum.shrapnelgames.com/showthread.php?t=41970