Whenever I have exited Cidhna Mine with Madanach and his gang they absolutely demolish the town guards. What gives? Are these special, nerfed guards that spawn for this quest? Or are Madanach and his allies just super OP?

The Forsworn are wearing their special Plot Armor, that's all.

The Forsworn timed it so that that guard has just changed and the "Milkdrinker" shift have taken over !