Reply one month later but figured I'd chime in since I'm about to head into the final dungeon on nightmare.

If you made it through FC/SC on Nightmare the 3rd will be no issue for you and it sounds like this is your first playthrough. It's by far the most balanced and fair of the 3 on Nightmare. The prologue is also not even remotely as awful as SCs, and overall I have not had any major issues. The strategies from SC mostly still apply. Earth Wall will still be important and you should keep an Earth Wall user in your party at all times. Kevin was my choice and probably the best since he also has Grail Sphere, he was a dedicated support character for the entire game. He should have Auto-CP items equipped for basically the entire game if you have the carryover bonuses or later once you get them. The best Action quartz available should be considered mandatory as it was in the previous games. Getting your characters buffed with Clock Up and debuffing with Clock Down makes a massive difference for much of the game but very late in the game some bosses are immune to any form of debuffs. Late game though you have more than enough tools to overcome that though. Debuffs in general such as the Def/Str down craft for Kloe is very strong for much of the game due to how high enemy stats are.

There are also 2 new characters from the previous games who's names I won't say to avoid spoilers but they are the last characters to join you and are very powerful. One is hands down the best offense arts user with zero line restrictions and the other is a monster physical attacker with hilariously low delay on their crafts.

Unlike the previous games there is also some optional rare weapon/armor drops available after a bit so it's a good idea to try and fill out the bestiary and look over what enemies drop. Especially for physical characters these weapons help out a lot and some are much stronger than what is available from chests or the shop for awhile. With armor keep an eye out for the Ninja gear. I used it for a huge chunk of the game because the huge agility boost and extra speed are extremely useful. The Ninja Tabi footwear in particular I'm actually using on a few physical female characters going into the final dungeon due to there not really being better options available compared to male characters. These rare drops can for the most part be farmed pretty easily though with just dumping S-Crafts and reloading if you don't get the drop some have awful drop rates especially late game though.

Lastly on the topic of S-Crafts there is a water pool on the far west side of the center of the garden that will let you fully restore CP after winning 5 battles. I honestly can't recall if this is available from the start of the game but take advantage of this especially if you are farming drops and later on it's very easy to abuse this by entering the 1st plane.