In my opinion, we need to use our Sizemore, speed and aggression.
Give Ya Ya free role, licence to run at them. Set pieces will be crucial. I could see us scoring home a& away from corners.
Ideally soak up their pressure, defend deep and counter attack in numbers as we have done countless times.
Make them worry about us, they won't like our size and pace. They don't have anyone who could deal with Aguero for speed, Negredo will be too strong for Pique, Biscuts will not know what to do with Ya Ya.
High scoring tie, a red card for either side will end the tie, City would test apart a 10 men Batca, as Barca would tear apart a 10 man City.
Over two legs I can see Biscuts or Macherano seeing red, and us scoring at least twice set set pieces. I may severely have under estimated Barca, but I do rate us highly enough to win.
Pace & power is the key to beating them. See Bayern 7 Barca 0 last year.