Basically to boost your elo in League of Legends you have to know about the actual meta and you have to abuse it. Like you have to ban certain champions for example : Riven, Kassadin, Dr. Mundo, Eve... etc. If something is not banned you pick it up and win games that's how easy it is. If mundo is open you have to pick it because you can carry a game alone with it, same goes with riven rengar.
Moving on from picks and bans. Another good thing to carry your elo higher is understanding the game. If you win your lane and you got 2-3 kills and a really huge CS lead, then you move to another lane to force objectives. For example if you win toplane and you took the tower, you go back to your lane push it up and you roam mid to try to kill the midlaner, then you force his tower and possibly a drake. Easy global gold lead can turn the game to your favour.
Placement matches are important if you want to get a good MMR (MatchMaking Rating) to simplify it, you win your placement games you get into a good league /division and allows you to division jump. Division jump works easy, if you have a good MMR you can jump from 5->3->1 divisions in every league.
It's the fastest way to reach a higher ranking, and it eventually allows you to play with better players = less feed, more wins.