If you play in MBL mode you have the option of, at the start of each level you choose to play, to play at level 1, 2, 3 or 999. Select level 999 for each of the initial 6 levels, this puts the game into the highest difficulty possible but don't worry, you can use continues.
I'm not certain whether or not this has to be done in one playthrough, so I would play it safe and go for this in one go.