Re the above...
Hence my incredulity that it would involve anything permanent. Had the Allagan devised such techniques, we would likely know of it by now. I think Gaius is probably just killing the hosts with his little posse. The precedent set by the WoL and later Thordan re Igeyorhm probably has Elidibus intrigued, though he doesn't seem to be the fearful type. I suspect he will be the most powerful Ascian we've seen to date. I would not be surprised if he has not begun investing in some counter-measures. Remember, after all, that he says to Varis that he is not too concerned if some of his black robed brethren are slaughtered and would not stop him. Perhaps he is already aware of what Gaius has been up to.
The last point I will make is that the overlords possess great power. As shown with Nabriales and Moenbryda, this extends to great physical strength, but more importantly, they possesses a devastatingly powerful arcane arsenal. Nabriales, and Igeyorhm/Lahabrea (and then the Ascian Prime), would not struggle even a little to crush a mortal foe who was not the WoL. It has long been my suspicion that the WoL has a much stronger concentration of "echo" powers than anyone else with the Echo (although there are probably some others who come close), upon which the Blessing of Light is then added on top. This is why they can withstand such beings, as well as eikons, void lords (think Forgall and his death magic), and such. Even the lesser Ascians, like the one in the SMN quest, can pose a significant threat, and the one in question only balked when you became attuned with Bahamut's aether traces, which in itself is a significant power. I'm also not entirely convinced the Blessing of Light was suppressed, so much as Midgardsormr made you think it was, to get you to rely more on your own abilities and wits, but this is an aside as my view is that the concentration of Echo in the WoL is really the source of his power, with the BoL as a cherry on top. What I am getting at here is that Gaius & co would need some impressive abilities to best an overlord, if that is indeed what the red masks represent; alternatively, he would need some means of breaking their connection to their "host", which is what I actually suspect is at play. Any number of things could then be the tool, but it's anyone's guess as to what. Lastly, I will hazard a guess that to witness their full power, we will have to see them in their "true" form, as all other forms we've seen are akin to avatars.