As far as you manage to get rank S, you'll receive 1 SNP. So you need a way to (almost) always receive the S rank.
So, I found an interesting way to get S ranks almost always still at story 8. This means you've finished about the first quarter of the game. But you can actually do it in other places too. The base cost is two air orbs (?). They speed up, so that's a plus.
Monsters on the Red desert are weak to air. Every time you attack with air, you get a plus on your grade.
I had Elena using the Air Cutter to kill a row of 2 or 3 enemies with a single blow, and Theo was using Aero.
Mostly because Elena was using Air for more time than Theo.
My other teammates were just ensuring the battle would last 5 turns to get another bonus. Or stealing.
Using this, I got grade S on almost every battle (my level 22 team with the Frontline equips hardly took damage and could kill any monsters with only two attacks, which makes a decently fast fight) - Until MP ran out: in front of the recovery spot
Of course, you can refit this to any battle later on the game.
As you can get some nice items on the SNP shop, it might be a good idea to grind a little this way.
Not only that, but you're saving and recovering TP. I found this by accident while trying to regenerate TP to finish mission 8.
Of course, it depends on how quickly you'll get bored. And on how often you kill at least 3 monsters with air skills.
If you know an easier or better way (other than buying with real money, ofc) fell free to comment.
