Ferran Torres struck a hat-trick, with Fermin Lopez and Lamine Yamal also scoring.
Flick said afterwards: "We have a difficult few months ahead and it is important that all the players show their best form and how good they are. They scored and played well.
"I am also happy because we came out very focused from the start of the match. I am happy for Ferran, he scored three goals. It is fantastic. He has returned after an injury and I am happy for him.
"I've already said it, it's important that everyone gives their best. Ferran had a great game today as number nine. I'm happy.
"It was a very important win for us. I'm very happy for the team, for how we played. We created opportunities and scored from the start.
"That's how we want to play, the team is always hungry to go for more. It's a young team and it's focused on scoring more."
On Pedri, he also said: "He's incomparable, incredible. One of the best in his position. He enjoys playing, having the ball. It's nice to see him play wherever he plays. He always plays for and with the team. I'm very happy that he's at this level."