Arsenal had the game going according to their plan (concentrated defense and counters) at least half way for the first 40 minutes (they missed their big chance).
At the end of the first half they lost concentration and did not get it back after halftime. That is clearly not enough to survive against Barça even on when the Blaugrana are not having their best day.
So excellent result without our boys having to burn too much energy.
It's not so much their concentration that they did lose. It's tiredness. Arsenal gave their absolute all in that first half, and before it was over they were gasping for air and therefore lost their defensive shape and intensity.
Once the 2nd half started I was almost 100% certain that we would win this game. Even though Arsenal had a few good chances (and a very good one as well) we had significantly more. A bit more coolness from Neyboi and Pistolero and this could have ended very embarrassingly for Arsenal.
God I love this team. They just keep winning big game after big game. And that's what I want to see.