I'm no S rank (just a lowly A), but I'd say for me the most important thing is situational aware. Even more so than getting kills. Know the maps and how to get around them. Know when to go in and when to hang back. When an enemy dips into the ink, be able to tell where they're going based on the direction they were when they went into the ink. And try to watch their splashing (though that's a lot harder).
As far as killing enemies, that all depends on your weapons, and is more about raw skill. If the problem is that you can't kill people even when you get the jump on them, part of that may just be that they're really good (as soon as you get hit, you can duck into the ink and swim away if you're good enough), but it also just might be that you need practice. Just like anything, practice is the most important part.
And get hooked up to play with gaf. We've got good people who will beat the skill into you by killing you over and over until you learn how to stay alive. I'm in the middle of the learning process right now as well.
