All of my tylos have done well outdoors down to -3C overnight. They are caged in a sheltered area with a deep bed of leaf litter and a deep water area, and they just disappear into the duff if it gets too warm, dry, or cold. I wouldn't risk much more than that, as this is just a brief overnight frost - if temperatures aren't coming above freezing in the day, that protection won't help. Despite being subtropical, almost all tylos are found in mountainous regions where temperatures can easily drop close to freezing. Those in Nepal [which may not be T.verrucosus], Sikkim, West Bengal, and Bhutan are close relatives of T.shanjing and in some areas may spend 2-3 months buried under snow. I would be more cautious with any species from south of China or closer to the sea, but even so, salamander habitats in Laos get very chilly as well.