There is a say, that a living being can get used to everything but hunger, that principle is often applied in many fields of life, for example spies are trained to tolerate different kinds of pain, it's not an exaggeration to say that practically they go through hell during training.
For undead, the definition of hell is the pain inflicted by the corrosive light, which was created by the light faction of the blue-sky galaxy, but even that can be taught to tolerate, that's why the best choice for a skin tailor to fuse is a human.
Because like that, their innate constitution will be similar to a human reducing their weakness to "holy light", making them feel just a little fraction of the usual pain.
Unfortunately for the skin tailor, Daimon didn't know which race will become his enemy in the future, that's why he made a bet and used both light and darkness as the base for demon light, then added he fire and lightning which are known to be extremely destructive to the mix.
