The best method to achieving this would be to unlock and use Maria on stage one. Grab the Phoenix subweapon from the second torch you pass and collect enough hearts to use the item crash (
) twice on the boss. One more attack with the Phoenix subweapon attack (
+
) and the boss should be dead. Also note: in the part of the stage with the giants, simply use the phonic subweapon attack a few times quickly to kill them fast.