Luckily, I was able to get away with not needing Master difficulty and Ex every single live in the game with Pro even the solo ones. Of course, you need to plan yourself with the best coordination of outfits/accessories, maxing your burst/omoide stats, and having a level 10 fast song. Plus, S-rank DL live costumes are pretty useful with their more hearts and perfect combo bonus.
There was one live where I did use Master difficulty (but I bought the target support z item from someone's advice to help me out with the perfect combo). Remember, it's a consumable item so you have to buy multiple if you want to keep using it. Though, if I grinded the burst level for my 3rd member I may not have needed Master difficulty. But, point is you can use Master difficulty + target support z as back up if there's a live that's giving you a lot of trouble and you're so close winning.
EDIT:
Just to update my progress, I got S-rank both Chihaya and Miki and got them 5 hearts (Haruka is almost at 5 hearts). I also Ex all lives even the DL lives, All Star, and solo lives (Chihaya and Miki for now). (I'll confess, I did buy a few DLC money to help me out a little since 10 million fans per idol takes awhile.)