Cold does seem to be a major factor. I think Minna said that trolls don't endure cold at all well, needing to hibernate ( I think that was what was going on with Leaftroll, it may have survived in its cocoon in a more sheltered part of the building when the ones Lalli punched out succumbed to cold and exposure? Some of the fans have theorised that since the grosslings appear to need water, areas of extreme heat and drought may also serve, such as deserts. Rainforests would be hopeless to retake.
Personally I rather like the idea of survivors in our Australian deserts, with their extremes of heat and cold. There is water if you know where to look, and once there were too few humans to tap the artesian waters as extravagantly as at present, the aquifer should regenerate quite fast.
I can imagine people taking refuge in places like Kata Tjuta, Uluru or Mount Conner, or the caves under the Nullarbor, where there is water in good plenty. Mount Conner maybe not, because although it is quite defensible, and has water, as well as nearby good resources of minerals and wild food plants, even in our world the place is famous for the hostility of its landspirits. Though a lot would depend on whether or not marsupials are sufficiently 'mammals' to beastify. If they are, we're all doomed!