You can wait around forever for these guys to spawn or you can Kill all the abandoned Heretic pets and he will spawn at around pos 1840, pos 410. You can kill the lot of abandoned pets at once since they all spawn in the hut at pos 1104, neg 844. Good luck.
The heretic prophet can be found in the forest anywhere from the bridge to erudin town gates. The guards will not help you if attacked by him. He walks around killing creatures and young erudites.
They con blue to my 6th level Shaman. The first three I came across killed me or drove me back to Erudin. Then I got smart and cast Fleeting Fury on myself before casting Frost Bolt on them. I was able to defeat the next two I ran across in this manner, with the loss of three health bubbles each time. I plan to continue the hunt often ;-)
I'm a Mage of the 9th year. These boys are very green to me. They must be 3-5. Check out: http://www.geocities.com/ficticiousname9/zone/Toxxulia_Forest.html