Well i am not a decent League Of Legends player.I am just normal or even below , thing is the game is made in such a way where you cant wins games alone , at least not frequently so you can rise on elo alone.
I have carried some games with Irelia on top , i was so powerfull at late game and i turned the game arround but this is hard to happen because of team doesnt communicating etc.
I stole baron with sona before 1,5month or so (we killed them afterwards thanks to my ult) my team were ahead and we could go for inhi turret but my team decided to split and farm on top/bot/mid/jungle w/e.When enemies respawned my team-mates were roaming , got killed one by one and we lost.
Just an example.
But i guess you guys know better since you are better players than me so i am gonna stick to your advices.