Multi Index Sorting in Pandas
When sorting by a MultiIndex you need to contain the tuple describing the column inside a list*:
In [11]: df.sort_values([('Group1', 'C')], ascending=False)
Out[11]:
Group1 Group2
A B C A B C
2 5 6 9 1 0 0
1 1 0 3 2 5 7
3 7 0 2 0 3 5
* so as not to confuse pandas into thinking you want to sort first by Group1 then by C.
Note: Originally used .sort since deprecated then removed in 0.20, in favor of .sort_values.