I think the idea is that the power now comes from within Shulk instead of the Monado.
Even at lv 70 I'm still having trouble with some bosses, if I hit another difficulty spike I'll have to take a break, there's no way I'm grinding at this point.
Gems can make a huge difference and it's not that hard to make good ones if you have a few characters with high affinity. Plenty late game quests also reward you with some very nice gems.
Trying a different team also got me through some though battles. For instance, Riki and Sharla (with heavy armor, in case she draws aggro) combined with someone with good DPS worked wonders for me. Forcing topples also works on some bosses. Melia's pretty good at it. Plus, it's funny to see her topple huge enemies with that clumsy kick of her's.
Anyway, I'm going to finish up today. I did the majority of the quest, hunted down a bunch of L95+ monsters and saw nearly all heart-to-hearts, and that's enough, I'm done. Well, after I beat the final boss, I'll be done. I'll get right to it after I finish my cup of tea. I hope.