The heretics are on a small island in Kerran. Look for the docks in Kerran, jump into the water and swim forward leaning a tad to your left. You should spot the small Heretic Island quickly. The zone isn't TOO big, so you shouldn't have much trouble finding it when you get into the water =]
To get the part for the cleric armor waste these pests in the front. You don't need to kill the ones in the back, as Maugarim's PH is in the front. A relatively well leveled Mage (Around 30th) can take a good majority of the island with a pet. Just make sure you have the cleric whoes after the armor with you to heal the pet if things go bad.