How do you become better at this game?

I hear all of these stories of people climbing to 5k with an underlevelled deck, people getting consistent 12wins in challenges and/or getting 20 wins. How do they do it? I've been playing since the release and I do tons of matches, whenever I lose I analyse the match and often the cause of loss is simply because his deck could outplay me. I also often show my replays to my clan and they usually tell me the same. I usually get 6 wins in classic challenges and about one in 10 times I get to 12. My pb on the 20 win challenge is 13 and I played it over 5 times. How are these people so good?