You really should be aiming for SS. The moves you get make a huge difference the further you go into the game.
Moves you should be getting from them in chapter 2 I think are:
Spoiler for moves, Daraba fight tips and 2-1 help:
2-1: Satsuki's 500M attack
2-2: Nozomu's 580M attack
2-3: Katima's support move Final Velocity, appears as a 0M/0F 3 mana move. It allows you to have an extra turn with her team. However, you need 3 mana to cast it and she MUST be in the supporter position once the battle is over/won. Final Velocity is one of the best moves in the game and is pure gold the deeper you get into the game and when you try harder difficulties.
2-4: Nozomi's 720M/320F defense, essential for him to AR properly in Chapter 3. I think you get 2 choices though in this one. If you do, get Katima's 8%M/500F attack (I think this is available just before the Daraba fight and the Nozomu one as an SS reward).
Other moves you need by the end of Chapter 2 are:
Satsuki's 350M/360F attack
Nozomi's 600M defense that protects all slots (if it was available in this chapter)
Nozomi's 700M defense, makes the Daraba fight manageable
Tip for Daraba: Use a Nozomu/Nozomi/Katima team. Alternate Nozomu and Katima between Attacker and Supporter EVERY turn. Make sure Nozomu's support move for the battle is Celestiary (the 10% heal and gives his other party members mana) and Katima's is Weaken (has an A mark, denoting she reduces the attack power of her opponent in that slot). Those two moves going back and forth will prevent Daraba's support move ever going off, because it attacks all slots and will 2 shot a team with ease.
To get SS in missions, construct a Sands of Time early and kill an opponent team in one battle. If you're having trouble with 2-1, try this:
Send Nozomu/Nozomi/Green team North and Satsuki/Katima/Red team West. Pick Nozomu's 20% heal + attack buff move and Satsuki's support move (Vanish) when you take the two towns along the way. (Nozomi's 700M defense isn't needed yet and is a distraction to make you pick the wrong thing). Build a mana converter at the town the Katima team takes. From memory, mob pack 10 can only be beaten if you have Nozomu in Supporter with the 20% move on Turn 1 then Attacker for the rest of the battle. Katima should always be in Defense in Turn 2 unless a Red supporter is attacking defense, in which case Satsuki should be in Defense. If a defender in West opponents has no F defense, put Red in Attack until it's down, then back into Support. If they have high F defense, put Red in Support to take out their Attacker.
Tip that may help - Generally, put Nozomi in Supporter on the turn you win. She will do her 25% heal which will lessen the need for recovery.
Send Katima team into the final point before 474IT with this lineup:
Attacker - Red
Defender - Katima
Supporter - Satsuki (use her new Vanish! Only this spell can counter the Red Supporter's 720F all slot move, which will annihilate your team otherwise)
Use that lineup and you'll win this battle without taking a single point of damage.
That strat should get you in at 428IT
I tried just playing this through with S ranks. In the 3rd chapter, I hit a wall. SS ranks will make life much easier.