This guide describes the formula that I found the most useful for achieving that sweet, sweet SS rank. Use this to accquire more of those nice materials or become friends with more characters. SS is the highest rank, and will give you a whole lot of XP, so be sure to go for it each time.
First and foremost, despite time being a factor in achieving the SS, I would advise against going straight for the boss; be it a large abnormal or a bizarre titan, the latter only appearing if you've completed the story. If you let the boss start roaming the fields, sometimes auto-firing bases can take them out and finish the mission before you can get that rank!
1: Side Missions
During your missions, you will see green flares go up over the battlefield at various times. One side mission will begin near the start - go for it when you can, but always be aware that your time plays a part in this. To help going for that SS rank, you MUST complete all side missions, or else it won't work as well.
The missions can include eliminating a single abnormal, or eliminating titans, establishing a base or escorting someone. They can be failed if the person getting helped or escorted is killed or you don't make it in time.
When the time is beginning to run out on side missions, they will turn into urgent requests. They have a shorter timespan, and are the side missions, but the game is just giving you a warning.
To recap: complete ALL the side missions throughout the scouting session.
Still being aware that time is a playing factor, slicing at least 95% of the napes across the battlefield will contribute to your rank. That would mean leaving at most 4 titans alive, which, if you do, it's still worth it killing them unless you're on Inferno mode, which, if you are...how are you achieving SS anyway?
Jokes aside, it's also worth to note that complete eliminations will bolster your chances of SS. This means you don't get grabbed, or don't miss an attack. The easiest way to do this would be to get the "chase down" skills, and slice off the limbs and then go for the nape, or, if you're brave enough, just head straight for the nape as well! Superior eliminations are also good enough, but efficient or lower doesn't help cut it.
A way to get some easy complete eliminations is to do sneak attacks. Being undetected and looking through your telescope by holding F is basically a one-hit kill, provided you get the timing right. You could also just target the titan from far away and zoom in, still undetected. This removes the chance of getting caught while you're in the telescope, but there's a lower chance of sneaking.
3: Dire Eliminations and Escaped Titans
Once enough time passes, after you complete enough side missions, or you kill enough titans, a dire elimination will show up. They can either be a buffed large abnormal or a buffed bizarre titan. All types of bizarres and abnormals are equally dangerous - they have huge amounts of health, have good reflexes, and are very fast.
To ease the pain of taking down a DE, you will want to hit the weak spots (obviously). Sometimes if you have good enough gear (Inferno set) you could heat straight for the nape, but it's still better to get the green glowy bits to weaken the titans. It also doesn't matter if you hit the nape or capture them, either, just that they're off the field.
Then, there are sometimes titans you have captured previously. They'll show up, and they'll escape, and your goal is to recapture them. Usually, I've found that a Scout will get on the job, and sometimes they've even recaptured them, but it's better to handle it yourself. I usually leave this until later, because there's a chance the Scout will get them while I'm handling the DE, but they can get away, so I honestly wouldn't recommend it. I'm a hypocrite.
Doing both the DE and the recaptures will give you more chances at the SS. Go for them.
4: Clocks on the Wall and the Watches on the Wrist
So, I've mentioned before that time is a factor in getting that SS, but it's not that big of a player, I've found. I've easily achieved SS by eliminating all titans, doing the DE and recapture, and all side missions, and taking down the abnormal/bizarre, and I still got SS and that motherload of XP and materials.
Still, it's best to be aware of time. As a side note, at the start of every battle, check the victory and failure conditions. The failure conditions can involve time running out, and failling the mission guarantees no SS rank, so watch out for that.
5: Spoils of War
So, apart from bragging rights, what does SS actually earn you? Well, to upgrade your gear you've definitely found some that will require a few rare materials. I myself have been grinding the last story mission for those giant crystals and red-hot crystals. And you might have noticed things like S-Rank Steel, which can only be achieved at S rank. So certain materials can only be obtained at higher ranks, and SS guarantees you'll get some rare nicities. (Although I haven't got SS in the final chapter yet...)
And then there's the XP. If you're like me, and would like to get 100%, you would be trying to max out your CPC, custom player character. Well, if it wasn't obvious, SS will get you the highest boon of XP possible, and this also applies to Another Mode, when you're trying to get Eren to max level. The pain...it hurts.
So that's why it's so worth going out of your way to do everything in the mission, building bases, saving people's lives...you get a bunch of rare materials and XP to become the most ultimate titan-slaying badass, above Levi. Well...maybe. If you're still trying to figure out multi-strike..
I've now explained the reasons why and the reasons how (English, I know) to grab yourself the SS rank and earn your Wings of Freedom for daily life. Of course, both of these apply to story and scouting missions, so change of fate in story mode helps as well. And besides, who doesn't want to spend HOURS trying to save Hannes... I mean, seriously, the mission and the side missions are all time limited, does anyone else find it extremely hard?
Enjoy your CPC at max level, enjoy your journal filled with stories of success, and realise that actually at the end you die, so in a lore sense it's all for nothing. I mean, you don't even get to get revenge on the Armored Titan for crushing ma and pa! Anyway, that's it.