GK: Iker Casillas
LB: Ashley Cole
CB: Thiago Silva
CB: Vincent Kompany
RB: Philip Lahm
CM: Xabi Alonso
CM: Andrea Pirlo
CM: Yaya Toure
LF: Gareth Bale
CF: Cristiano Ronaldo
RF: Arjen Robben
BENCH:
Schweinsteiger
Hummels
Falcao
Aguero
David Silva
Chiellini
Neuer
The point of this team would be to defend well and counterattack effectively using pace, the best way to beat Barcelona. To that end, it is designed largely to allow for swift counterattacks. Kompany and Thiago Silva are particularly adept at starting counterattacks from the CB spot. Lahm has a fantastic long pass to start counterattacks, and Cole is a good passer. All three midfielders are very solid defensively and, combined with the defenders, would be quite hard to break down. What's more, they'd likely dispossess Barca's midfield more than most teams, allowing some easy counterattacks up the field. Xabi Alonso is probably the best in the world at making a long pass forward to start a counterattack. Pirlo is probably second best. Yaya Toure is quite good at this as well, but also will make a lot of conventional through balls further up the field. The forwards are all about pace. Barcelona plays high up the field and does not have the fastest CBs, so they are weakest against pace. Ronaldo, Bale, and Robben all have insane amounts of pace, can dribble past their man if given space (ie. in a counterattack) and can all score quite well when through on goal. Ronaldo would be adept at poaching goals Bale and Robben would create by beating their man off the dribble. In addition, they'd be hard to mark, as all of them can float around the front line to some degree.
Thus, the defenders and midfielders would create swift counterattacks as quickly as possible. The attackers would use their pace to jump on through balls or dribble past their defenders during these counters. Barcelona is also weak on set pieces. This team would exploit that as well. First, Ronaldo and Robben are quite good at direct free kicks. Pirlo and Xabi Alonso are fantastic at taking indirect free kicks and corners. Much of the team is capable of scoring from those, but Ronaldo is particularly good. Lahm, Yaya, Alonso, and even Ronaldo are very adept at the long shots that can come after set pieces.
The bench mostly continues the philosophy. Schweinsteiger would fulfill a similar role as the other three CMs if one were injured or tired. David Silva gives more of an AM option if a goal is really needed. Hummels is another great defender who is adept at starting counterattacks. Chiellini is similar but can also fill in at LB, while giving an even better set piece option. Aguero is another guy with great pace and goalscoring who could play anywhere in the front three. Falcao is more of a change. He doesn't have the pace, but could punish Barca on set pieces and poach goals from the pacy wingers. Neuer is just another great goalkeeping to complement Iker.
While I think this team would have a good chance of beating Barcelona, I think it's a real testament to Barca that I find it exceptionally hard to come up with a squad that could beat them from the entire world outside of Barca players.