Fuga: Melodies of Steel

There are three factors that determine your battle rank: turn count, damage received, and technique. The first two are straight forward, make them both as low as possible. But the last one can trip you up because it's based on how you fought during the battle. Specifically, it uses this rating system (you can find this info in the battle results screen by pressing the View Details button):

• 100 points per normal attack
• 1000 points per skill
• 1000 points per weakness hit
• 2000 points per status debuff inflicted
• 3000 points per link attack

So if you don't use anything other than a normal attack, your score for this section will likely be low (especially common in early new game+ where enemies die in a single hit). There is some amount of leeway given, but even with S ranks in two areas it is possible to not get an S rank if the third section's rank is too low.

I was able to get this on my first run naturally, but if you need to farm this later, just make sure you use at least one skill and hit the weakness even on the easier battles so that you don't completely tank the technique score.