Having faced the Stone Skin technique once before, three months were more than enough for Mi You to come up with a countermeasure for it. In fact, he came up with the countermeasure for Stone Skin during the very night he faced it.
Tai Shan strike, an attack that used high-speed oscillation to cause discoordination between the skin and flesh, causing the skin to crack and break away from the flesh underneath. While the undead had living statue abilities, they were still creatures of flesh and blood.
Being essentially skinned alive, even an undead wouldn't be able to endure that type of suffering. The undead blood aura would quickly allow her to regenerate, but Mi You would simply use Aura Summation to petrify her skin, then break it off once again. This was nothing short of the cruellest method of torture, but to Mi You, there was no need to empathize with an undead, especially not one whose mouth stunk of human blood.