Ogres certainly do not have a way to one shot mages at such a range
placing runes on a typical path can kill even an invisible mage. And you can force the mages to take that path by continously walling off entry spots (s12 to 11 on gow for example, followed by the s12 to 9 spot, leaving only middle as the path to come from to reach 11, as you can put the farm at the top of s9 too ) Granted it's not a technical 1 shot kill, the odds of them stepping on another in a tight space is pretty high. It takes 2/5 of the runes to kill. so it can technically kill multiple mages in a single cast. And the odds of you microing a mage back after just stepping on nicely placed runes requires a very slow game speed.
Other then that your point on the Exorcism is fine, gold cost or mana cost reduction is fine. A mana cost reduction will allow you to 1 shot a death knight after spotting it with Holy Vision with the same paladin.