Hearthpwn

To put it in it's simplest terms, you need to overthink everything.  Spend a lot of the time at the hero selection screen, studying the tribes and how each hero can or can't win based on the selections.   It's always best to come in with a plan, you can't force that plan, but it's always best to have one.  

You also have to really understand in situations how either leveling or not leveling benefits you, winning is all about those big power level swings, and sometimes the only way to do that is by leveling, but you also have to understand in that situation if leveling is safe or will leave you dead.   And in some cases all you can do in those cases is pull out a top 4.  But sometimes leveling is completely pointless and just wastes your turn for no reason.

You obviously need an understanding of what's possible in the game, and that's just too long to get into.   It's a lot of trial and error, learning from your losses and yes looking at other streamers play.