what would be the weather/temperatures we can expect in late december at badaling great wall

According to the previous weather statistics, it was 25 F to 40 F during that period of time. There will be mainly sunny days. The climate will be cold and dry. You are suggested to bring heavy overcoats, sweaters, pants and other winter clothes.
Generally speaking, there won't have crowded people in December because it's not the high tourist season. If possible, it's advised to visit the sceinc area on weekdays, not on weekends.