import pandas as pd

mi = pd.MultiIndex(
    levels=[['group1', 'group2'], [1,2,3,4]],
    codes=[[0,0,1,1], [0,1,2,3]],
    names=['group', 'id']
)

df = pd.DataFrame({
    'col1': [400, 300, 200, 100],
    'col2': [10, 20, 30, 40],
}, index=mi)

max_idx = df.groupby(level='group').idxmax()

ret = max_idx.apply(lambda d:d.transform(lambda d:df.loc[d]).stack(), axis=1)
print(ret)
#       col1      col2
#       col1 col2 col1 col2
#group
#group1  400   10  300   20
#group2  200   30  100   40