#1 Atletico Madrid's ability to counter-attack
The ability of the midfielders to play centrally, and wide will be the key tactic to stop Barcelona. When Barca play through the middle, Atleti's midfielders including the wide players will stay central to provide defensive cover to their centre-backs.
When Barcelona go out wide, Atleti's wide players, along with the central midfielders go out wide to support their full-backs and block passes coming in towards the middle.
As the game goes on and Barcelona players become increasingly tired and desperate, Atletico Madrid will find more and more space to counter-attack. They have a lot of pace in the form of Angel Correa, Antoine Griezmann, Yannick Ferreira Carrasco and Saul Niguez to make Barcelona pay.