As someone who has been at BV with 0 LP TWICE this season, I just got into B2, the highest I've ever been ranked. I feel like I'm on pace to make it to Silver+. I pretty much main
but that's not why I win. I win because I've learned not to flame my teammates all the time. I've learned that it's ok to let other players die if I don't feel comfortable fighting a fight. I've learned to get carried. I've learned how to stay out of trouble if I fall behind so I can still be useful to the team. As a marksman main I honestly watch a lot of Challenger level solo queue, I listen to what they say about positioning and certain things to look for when playing (imaqtpie and doublelift are two of the best imo). Basically being more aware all around. Learning to up your CS is a great way to get ahead in lane without necessarily getting a kill. If your support gets a kill don't get mad because chances are you got an assist, and now your support is more fed than their support which is never bad. Knowing when to pick a fight is also key. If that
whiffs on his hook, there's a good 25-40 seconds he's useless.
There definitely are certain champions that are more powerful than others. That's the state of the game all the time. So avoid picking things you aren't familiar with just because some website said it was a counter pick. I have over 300K mastery points in MF, I've played against pretty much everything, so I don't worry too much about what counters ME, as much as I worry about what counters my team's primary win condition. Also try to pick into what the enemy picks into. I.E. they pick an aggressive support, don't pick a support that has to stand in the back and just hope you survive to late game (Soraka/Janna), pick something equally as aggressive.
The main thing though is, don't flame. Learn that some games you are going to get behind, a teammate is going to get behind, that is the nature of League.