For the most part, all of us who come here asking for help getting out of "elo hell" have a good idea and a lot of technical knowledge of the game. We know all about many different champions, what items work well on them, why they are famous/infamous, and how to deal with them in a laning situation.
However, the biggest problem that I find many of close to or above 1200 face, and the outcome of many games, comes down to who can out-do the other during the team fighting phase. You can create the biggest early-game advantage ever, but if you can't do well in team fights, identify what to do, and to act on the knowledge you have accrued from the laning phase, then you will not be successful.
If you're having trouble, just remember a few things:
Keep you wards up
It's not the sole job of the support to ward everything. Sightstone is not an excuse for supports to not buy wards, because they might need a pink ward. The more you know about an enemy, the more you can do about it.
Wards may be pricey, but they are worth it. The biggest mistake out there right now is getting caught out of position. It is harder to get tanky, so even guys who like to play tanky champions, such as myself, cannot get caught out of position, or they start suffering. Armor and magic resistance have been made generally more expensive, and armor penetration has received a general buff with Black Cleaver. Wards and good map awareness are the key to any good game now. Don't use the excuse of saving money for a big item. Even one ward, one known enemy position, can be enough. You don't know if you'll get the money for that next big item if you don't get that important ward.
Don't berate your teammates
Your team is your biggest weapon. If they do something stupid, don't belittle them for it. Just move on, and make sure it doesn't happen again. If it does happen again, then now you are aware of it, and can deal with it. Your time will be better spent focusing on improving your mistakes and identifying weaknesses in the enemy defenses than in raging at teammates, regardless of how bad he/she is.
When in doubt, focus the ad carry or other squishy
When it comes time to team fight, it may be hard to go after the one who is causing all of the damage, but regardless of who you are playing, if you can hit the ad carry or other squishy target, show no remorse. Don't over-extend in the process; depending on who you are playing you probably shouldn't be dying, either. This could be hard to do, and there is always that "but what if?" argument, but when get the chance to shoot at the ad carry, take him out, now, before he can take yours out.
As an example, in my last game, I was playing Soraka, and a huge team fight broke out near the enemy inner mid turret. The enemy Anivia and Miss Fortune threw down their ultimates onto both of us, and most of us took a hard hit. With the entire enemy team right there, my health bar was dropping fast and I knew I was going to die, so I dropped exhaust on Miss Fortune so that she would not be useful any more. This put some pressure off of Tristana and Lee Sin, and we managed to chase them all off with only Rumble going down.
In another game, our Evelynn player's sole existence was to get into the enemy team and take down Graves. By nuking the enemy team over and over again, she managed to pull us from a 1/15 game to a victory. By preventing Graves from being useful, it bought Maokai and I, Soraka, more time to get some items and gave us an opening to get ourselves in good shape. From there, we snowballed them into the ground.
Understand your place in team fights
If you're a tank, lead the charge. If you're a tanky-dps, follow in and target the carries. If you're an Ap caster or Ad carry, hang back and wait for something to happen, and when it does, throw out everything. Every champion serves a slightly different role in team fights. Figure out which yours is, when you should pick that champion, and then go for it. Don't initiate as an ad carry unless you have something like an Ashe arrow. Don't just sit back and wait for something to happen if you see an opening with an initiator. Experience is the only cure for this, and you will figure this out as you play more champions.
Don't go in half-asleep
This has been said many other places, so I'll just leave you with that. Don't go in tired, or you will suffer. Go take a nap, go take a walk, go play a Dominion game, go do something else. The world will not end before you get back on.
Realize you will not always win
This is the biggest thing by far. Even if you are the greatest team fighter ever, even if you are the best at what you prefer ever, it may not be enough, and that's okay. If you lose, that's okay. You still learned something. A victory teaches you that a strategy worked. A defeat tells you that something didn't work, and in time you will learn to pick out what didn't work and act on it. Did you initiate when you shouldn't have? Did someone buy Trinity Force when Frozen Mallet would have been a better choice? Little things like that add up in games, and defeat is how we learn best. You may not like to lose, but very little in this world was created by an accidental stroke of luck. You might get lucky, but don't hold your breath. What you can do is improve yourself, over time.
If some dcs or trolls, then deal with it and move on. There's no point fretting over how good of a player you are and how much of a troll everyone else is. There is only one person who you can affect: yourself. Get the **** over it and move on.
I hope that you can take away something from this and that this will help you identify whatever problem you may have at the moment. I know there has to be something that I left off this list, and I will add it on if I need to. Thank you for taking the time to read this, and good luck out there.
TL;DR, read the bolded text, improve, start winning more than losing
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