This answer tries to give few insight on how to increase your winning chances.
First, there is no such thing as Elo hell.
But then, why am I stuck here when I do alright and my allies feed like (Elo) hell?
Watch the following video, it gives a good insight of what you should/shouldn't do. They are not dark magic advice but they are very important:
Youtube video 2
Few more points:
Never flame, always stay positive
If you flame etc your allies, you will most likely lose the game. Don't even try to be constructive, just don't point out mistakes. Either they will know it was a mistake and then it's useless to point it out, either they won't want you to tell them.
If the flaming comes from someone else, just mute that person. Reading and typing will never, NEVER, make you play better.
Never surrender. You might have a big disadvantage, but if your team starts playing like a team, you might have a chance.
Become the leader
In season 3, I managed to climb up to GOLD 3 by playing support mostly. Because I was always calling the objective, warding properly to be able to secure drake etc...
Don't under-estimate objectives and vision. And if nobody does it, YOU WILL have to be the leader and make the call. A team needs a leader, if nobody is, you will have to step forward.
Try to keep up with the drake, buff timers. If you don't have the vision when they die but you suspect them to be cleared, right it down so you can challenge it next time.
Don't blame others, blame yourself
Even if during a game you are at 5/0 and your allies are feeding a lot. Don't blame them, blame yourself. You can always play better. Maybe you could have ganked more other lines, maybe etc... Try to see what could have been possible to do to increase the chances.
This point is actually the most difficult. Sometimes, you will be 0/3 on your line because the jungler got you. You can ask for help from your jungler. But don't blame him. I'm pretty sure you could have saved a kill by warding properly etc...
It is frustrating but if you always blame others, you will never improve on the points you need.
Considering the champions
There are slightly more powerful champions, sure. Depending on their role, how they are played etc.
If you get countered, then ask your jungler for help right away, play safe. The goal is to not feed him.
In general, it's better to play top tier list champions. But the most important thing is to play a champion you play well, you are used to. There is not point playing a champion because it fits your team if you don't know how to play him.
There are many more points to address but those are the principal ones people tend to forget/skip.