now i understand where is permutation of corner on megaminx come from.
this only works on edge that is already oriented, and with 2 or 4 corners.
yesterday i tried to rotate the cube and permute 2 and 4 corners using D moves,
oh my god, the corners are oriented and the bottom layer doesn't messed up.
this is why learning 3x3 first is really important if you want to solve another puzzle like megaminx, 2x2, and big cubes, and skewb.