Two good teams, one alien. To put it short.
I think that Guardiola messed up the initial tactics, it's suicide playing 3 versus 3 in the back against Neymar, Messi and Suarez.
The first 15 minutes Barcelona should be up at least 2-0, but then Pep moved to a 4-4-2 and the game got tight. In the last 15 minutes Messi played the best 15 minutes of his carrier and won the game.