I guarantee stomping on his head is far from painless. Especially if you don't get it right the first time. Techniques like severing the spinal cord can be effective, but most don't do it correctly and leave the animal twitching in pain.
I've produced thousands of leopard geckos and thousands of other reptiles. Unfortunately, euthanasia of deformed or sick animals comes with the territory. When I put an animal down, I put it in the deep freeze which is set to about 0 F. They immediately go to sleep and then die. It is painless and the technique most all reptile breeders use and recommend. I also use and recommend the C02 method. But again, if not done properly, it is far from painless.
Is that your Tagu? Do you cool him in the winter? Do you think he's in pain when he is in brumation?