There are some impossible games because of 4 feeders but it's rare. Here's what I would advise.
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Have a good early game. Preferably kills/assists but having no deaths would suffice.
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If you see your allies are all losing, they may be on tilt, so this may be impossible. Adopt a mentality accepting this fact.
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Without being bossy, take the leadership role. Because of step one, you may have established a level of trust.
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In such a leadership role, remind people to upgrade trinkets, and frequently remind them to ward.
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Suggest losing lanes to build defensively.
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Try to spread your lead to other lanes, if laning, or focus on one or two lanes as a jungler but don't spread too thin.
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FARM. By God farm. You are likely the best player on the team. Sacrificing all of your farm to put your ally who was feeding ahead makes no sense. You died least, the lead is best with you. With a lead you can farm better. Get that gold lead. CS>Kills.
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Time the dragon/baron but know when you cannot contest them.
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Recognize objective opportunities and quickly direct your allies on how to mobilize to take them. Make good shots and it garners further support.
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If all else fails and they refuse to cooperate, you may be forced to hard carry. The above still applies. You first need to stop the bleeding by getting them to NOT feed beyond any hope of return. The second is that you need farms and kills. Lastly, you'll need late game scaling and MUST take a leadership role still and call the shots.
It takes experience but if you can bring organization to the chaotic realm of lower ELO you have one advantage the enemy team may not have. That's the best thing you can do. It's a strategy game but this isn't chess, and you CAN'T control the other pieces. You can only do your best and sometimes you'll lose. That'd be my last tip, you can't go on tilt like many of your allies may be on.
Getting carried is legitimately a skill because learning when and how to cut your losses when in a losing game and try to minimize the impact of your loss on the game to help your allies carry is something less skilled players simply won't have. Sometimes they won't want to learn but my advice would be this.
Right
[ ] Next time f***ing build defensively when you back.
[ ] I'd recommend you build defensively next back.
[ ] Could you build defensively next back, please?
[X] Hey guys, let's build some defense up, yeah? We're taking a beating.
My justification is this. The first one is rude. The second isn't wrong but people naturally think they're good, and as a defensive mechanism will be prone to denying advice out of pride. They have some dream of somehow still doing well. The third is polite and SHOULD be the best option but the pride thing still applies.
I think the last one may be best because first you identify with them and establish your team as a group. Then, you slip in your advice but make it seem like it's still their idea, not them being told what's best for them because they don't know. Lastly it shows humility by identifying with the lack of success, rather than you distancing yourself and giving advice. They may see it as an attack on their skill and you putting yourself on a higher spot, as irrational as it may be.