How Can I drop a column if the last row is nan

Try with dropna

df = df.dropna(axis=1, subset=[df.index[-1]], how='any')
Out[8]: 
     A  C
0  NaN  x
1    1  3
2    x  z
3    4  6

You can also do something like this

df.loc[:, ~df.iloc[-1].isna()]

    A   C
0   NaN x
1   1   3
2   x   z
3   4   6