The real nightmare for gold chevron rebellions is when they contain chariot units. Whilst you have a chance, the poor old AI in auto-calc has no chance. I've seen massive armies destroyed by a certain tribe of women charioteers...

Anyway it doesn't matter how much experience a unit has if it's vulnerable to missiles and projectiles. Next time you attack the settlement, bring in the catapults and plenty of archers and just pelt those barbarians with the midas touch from afar. They'll soon start to drop. Sadly the AI defenders love to run back and forth behind the walls and in range of your bowmen so just wait it out and don't attack too soon.

Ultimately, however, I find those crazy rebellions annoying and unrealistic.