S's are Too Hard to Get in Low ELO as ADCs Due to Today's Meta

Suck less, get more S.

Not true - ADCs, like all champions, get rated against their own pool of results for the particular champion played. For example, if Lucian as a whole is doing poorly, then the player's results will be compared with all the other Lucians out there.

If you want to get an S on any adc. AFK farm the whole game. That's how I got level 7 kog'maw

Well, some information has leaked out here and there, and Rioters have answered some questions about the S-grades in the past. So, this is my best effort to answer your question, based on what I have gleaned.

Caitlyn is doing extremely well right now (as witnessed by the many nerf calls for her on the forums), so it's going to have to be a stellar performance on the Rift if you are going for that S. (Most of her S-grade performances probably have a huge KDA and/or minion count, for example.)

As for your closing question: as I understand it, it's against **all **the other players - but there is a weighting of the result via an aggregate of the factors that Riot considers relevant (relating to your current game) are factored in to determine what the champion's performance was. To put it another way, Riot creates the profile of what an average player playing your champion scores on the Rift or in Aram and boils it down to a single score, then your performance undergoes a similar reduction to a simple score. Then the two scores are compared via statistical analysis based on a normal distribution (a.k.a. a bell curve). As long your score places you in the top 10% on the distribution of potential scores then you can look forward to an S-grade of some sort.