You don't have to be at Level 50 in order to take on the boss medley, all you have to do is to get used to the bosses' attacks and dodge them well. Antasma X's bat attack is definitely the hardest one so make sure you get lots of practice and remember that the bats only attacks at the middle and bottom section so pay attention to the shadows of it.
Bowser Jr. is helluva easy. Beaten him on the first try. Just keep using Bros Attacks against him, and whenever he's out of his Clown car , hop in it so you can get back your Bros Attacks more quicker and deal damage to him at the same time.