Lucifer's fight with Milka was absolutely unfair because he was fairly not a match for her in that aspect. Despite this, she persisted to be obstinate, granted that the bond kept the emperor from explicitly ending her.
By closer examination, all of his defensive attacks went on without the aid of Mass Mutation, nor anything but his incredible strength and/or great speed. Although his last move wasn't convincing in terms of this fact, it was still rightfully convincing to him.
By experience, Lucifer was certain that smashing his wife like a sandwich would not kill her, experience he gained from the war he had at Qopsta and as well as from his colorful days with her.
'Not done with you yet, Your Highness.' thought none but Milka. Moving like a gradually dispersing sticky ink, she penetrated her way upwards through the earth rubbles pressing her down.