The Praying to the Gods achievement requires you to kill the fanatic 10 times "without drinking any potion which restores prayer" and without leaving the wilderness. Simple solution is to pray on the nearby Chaos Altar or to take the obelisk down to a less busy altar near Ferox.

However, do you think the spaghetti code takes into account Stat Restore Pot Share or butterflies? Could you cheese this one with a friend?