Don't know specifically about Challis, but down river in Salmon, the majority of the geese leave when everything freezes up. For the few that do remain....I would pattern them and then set up around the field they are flying to. If it's just a few small groups, don't use a bunch of decoys. From what I've seen, the geese land in those smaller fields and it seems you could just hide next to the field they were using and pass shoot. Lots of fencelines/brush, etc. Or come down to American Falls and hunt the 40,000 or so geese that are here from Dec-end of season!
Lance