Irene just gave her advice, and she even used Wendy's power, that is hardly interfering, that is all, Irene is a persona, she never used her own magic in that fight. Why diminish her student when she tries to get her to be better? And Haku only turned her into a doll because she gave her advise, why didn't he sense her power? Why didn't that weaken Wendy? Selene later said that Diabolos was pathetic after Haku's defeat. Irene could use enchantments on Selene which has been stated that Enchantments do not work on those who have greater magic than their own, and Irene was a persona, she is literally dead herself, and dead people are stated to be weaker than their peak, and i looked back at the scene, it just said that it would be practical for another dragon to kill her, not anything about her power diminished. And yet Neinhart oneshot Jellal in return, so.