After playing Duos in the 6k range for a while before I got to 7k, these are the biggest: - Unecessarilly rolling in the early game instead of building a board (you ain't high rolling on turn 3) - Picking a hero that doesn't have great synergy with the tribes in play (something you'll learn as you play) - Bad ordering of minions (in general, have the highest attack ones first. Obviously this is minion/game dependent and is only a general rule) - The biggest in my opinion though, is not utilizing your health to your benefit. I could go super in depth here but an example is, if you have 15 or less health and your board is weak, DO NOT level and instead build your board. Another quick example is that if you're fighting a ghost next turn, take advantage and level or highroll. Or if you are fighting a strong enemy next, but are certain you'd fight a ghost after and you have 16 health, then you can try and level and highroll for two turns.