Walkthrough

Defender of the Faith is a Side Quest in Velen

To start the quest you must find the old woman in the road between Blackbough and Crow's Perch. She is enraged the vandals are vandalizes the shrines and says that if its not fixed the god's will unleash their wrath. To complete the quest you have to fix three shrines.

After fixing the shrine by the old woman Geralt must travel south of Midcopse. At this shrine Geralt will encounter an Endrega Warrior, avoid its stinger as it will poison you. Sidestep out of the way and use a firebolt potion and keep wailing on it until its dead. Take note, it has a forward dash that will knock you back and stun you. Once you defeated the Endrega Warrior fix the shrine and make your way to the last shrine.

Keep going south to the next shrine past Duen Hen. Once you reach the shrine Geralt will have found the vandals, Heralds of Lightning. Speak with them and it will lead to some interesting dialogue. Once you get to the last dialogue choice which is (Not about to get involved) and (Can't let you vandalize shrines), make sure you choose (Can't let you vandalize shrines). If you choose (Not about to get involved) you will fail the quest. Once you choose (Can't let you vandalize shrines) you will get attacked by the four men.

From there you can choose to fight them with fists or with swords, I found swords to be easier. The strongest one is level 10, so its not too difficult. Dodge-sidestep out of the way of their attacks and use AXII to stun them, which will lead to easy critical hits. Don't let them gang up on you, just fight them one at a time. Once you have defeated the four men you will complete the quest.