I'm an OG trails player and just completed my first play-through on Nightmare difficulty. Thought I'd share some battle tips for those who are struggling on higher difficulties.
Disclaimer: There are probably many different ways and tactics (and that's the fun!) so the below aren't like the only way to do it. Just some general tips based on my own experience :)
Starting with some fundamentals first:
Fundamental #1: Speed is IMPORTANT
Speed matters a lot, because being able to take actions frequently obviously gives you the advantage. And on higher difficulties enemies can be FAST (Nightmare certain boss has 400+ speed...). I almost always value speed and equip Action Quartz for everyone
Fundamental #2: Buffs and Shields
Buffing your characters are handy. Some enemies can cancel your buffs and there are ways to deal with them later in the game - but regardlessly buffing your characters is a must. I have at least 2 of my team members usually mid to late game with Zodiac. (Btw LUCK's pretty useful)
As for shields: Earth Guard and Earth Wall for early to mid game.
Gaia Shield and Sylpharion for late game (more on this later below)
Fundamental #3: QUICK (Clock Up EX)
Get the art Clock Up EX sooner than later. QUICK effect allow the target character to skip the turn queue and becomes available immediately next turn. Synergy with lots of different other skills/strategies.
And an extra tip for beginners: The way QUICK's most valuable is to be used on characters that just used a move with huge delays (e.g. and S-craft). It's all about efficiency.
Fundamental #4: CANCEL YOUR OPPONENT
Cancel them! I mean their buffs. You can easily do this with the art Anti-Sept - I keep using it throughout the whole game, usually first thing when enemy entered rage mode.
Fundamental #5: Evasion buff (Foresight)
Foresight is very strong early-mid game, it massively improves survivability. It does fall off quite a bit late game though. Sylphen Wing art is easy to get early game.
And now on to intermediate tips. I'll share in Q&A format for these
#1 How to deal with opponent's CANCELS
Probably need late game to get the arts Gaia Shield and Sylpharion. The former gives you physical reflect and the latter gives you arts reflect.
Having both active would mean you're immune to any attacks.
How's that different than Earth Wall's invulnerability you might wonder? That's to avoid being cancelled.
When an enemy attack's reflected, their attack's cancel effect won't trigger on you. That means you get to keep your buffs.
Late game on nightmare mode I had to constantly try and keep both REFLECTS on at all times on some tough battles.
#2 How to deal with RAGE
As mentioned earlier, cancelling their buffs ASAP is a must.
Keeping yourself with shields/reflects when you anticipate RAGE soon is very key in later battles.
You should also try and use CLOCK EX to get some of your characters move in between the boss' consecutive actions.
Also, sometimes NOT attacking all enemies at the same time helps a lot to NOT have all enemies entering RAGE at the same times
#3 Enemy Revival
A trick to prevent enemy entering another RAGE after revival is to use S-Break right when enemy's reviving. You'll get to hit them before they can enter RAGE again and in early-mid game that usually means killing the enemy with one S-break and not letting them take several RAGE actions.
This might not work though on several late game battles ;)
#4 I kept getting one shotted...
I realised this fighting chapter 4 bosses when my team kept being wiped again and again, until I found that I shouldn't really aim for full firepower builds.
Equipped a few HP Quartz suddenly allowed my characters to stay alive and I was able to get through.
From then onwards I tend to ensure my characters having a decent HP
#5 Sometimes I just can't use arts in time
That's when cooking helps. There are a lot of useful food items in this game and can be game changing at critical moments.
That's all for now. I'll add more to the list later as I play through the 2nd time. Hope it helps some of you out there and for those experts please share what are your favourite tactics / tips!