Basic things to help you out
There are a few simple things you can do to help you win any battle yet no one does.
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Power in numbers. If you are out numbered it is normally better to run. Now I know there are times when you can win while being outnumbered one or even two. If you are standing mid lane by yourself and you see three enemies coming towards you just run cause chances are you will die.
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The element or surprise. Simply if you get someone who is unaware to you they normally will not have time to react. Yet everyone wants to just rush everything.
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Force the enemy to fight two fronts. If you have a big ass minion wave pushing bot go all 5 top lane and force the enemy to reply. If you have a really strong duelist who is doing well let them split because it will most likely take two to bring them down which gives you numbers advantage. I know me saying send someone elsewhere is directly conflicting with number 1 so it won't be the right thing to split all the time but if they need two or more people to bring them down then let them split and play safe until they respond to them. If they send two then you are in a 4v3 and you now have the numbers advantage.
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Force the enemy to play your game. Like number 4 but continued. You could also join that big ass minion wave and rush the tower before the other team has time to respond to it. Once the enemy is there quickly go somewhere else. Making the enemy make quick and unexpected discussions.
These seem like such common knowledge but I hardly ever see anyone outside of organized teams doing these things. I know these don't apply to every situation but they are general things that can help you get an advantage or come back from a losing game.