With such power and lethality, it would be hard for anyone to face her. She was someone not of this world.
But Wilkis was also far from the same man he used to be, he had already entered the desired grade of five long ago and had even sealed his mother when she went berserk and came close to harming her own servants in the meat mountain.
Being informed by Alfin beforehand of this matter and used new medicines to help her suppress her anger. When the day eventually arrived, Wilkis arrived at a proper time to stop her. Sealing her in one of his void prisons, waiting for a cure to be made for madness, or just waiting for her madness to die down before talking to her.
Something which might take years to happen for his mother was not a special case like Alfin, who had the advantage of a greater entity overlooking him. For Umbrel to regain her senses, it would take a lot of time. And that is if she does regain it.