Improving Champions

I was watching a LS video earlier today and he was talking/coaching a guy (who wanted to get better at the game) who just played Rammus and Warwick. LS told him to stop focusing on not messing up and winning as you are not focusing on improving. I also want to improve at the game as I am Silver 1 and I really want to reach Gold this season. How can I improve. For example, can someone tell me some champions who are hard to play which will generally improve my mechanics or macro or tricks which can generally improve my global knowledge. Please don't recommend improving one champion which does not require global macro or mechanics. For example, this same guy was sing LS's meta champ system which said Warwick was a must play for Bronze / Silver / Gold ranking yet when the guy asked how he could climb to gold, LS said stop playing Warwick. Can anyone please help me out? Thanks!
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