I've never had any trouble beating Barca, I find it harder to beat teams that dont come forward.
I remember i did a Master League with Valencia a few years ago and everytime i played Real or Barca i stuffed them but then Racing came along and either frustrated the hell out of me to 0-0 draw or they had one shot and scored and beat me 1-0.
IMO opinion, the best formation is a Villa Style 4-3-3.
With a tall post player up front (Carewand Ibrahimovi are good but Berbatov would be my choice because he is exceptional 1 on 1 and his touch is 99% perfect) then 2 speed merchants either side. It's not necessary to have a great passer, obviously that helps, but raw speed is the key (Aaron Lennon and Odonkor are perfect).
The midfield would consist of an AM, CM and DM. The Attacking midfielder being someone with good shooting ability, especially from distance. Speed isnt really necessary here but i can help (Gerrard, Lampard, Deco, Scholes, Stankovic etc are all good). The CM would a passer, nothing else really matters here (Xabi Alonso, Fabregas, De La Pena are good choices). The DM needs to be tall, good heading ability and with a bit of pace to get back if needed (Viera is undoubtetly the best in the game in this role but there are a few more out there, Essien and Mascherano although they dont have the height are good choices).
It would be a basic back four for with pacey passer at either side (Chivu on the left and Cordoba on the Right are good picks but in the Centre is where it matters most and Height, Strength and Pace are your No.1 priority (Rio Ferdinhand and Gallas are probably the ones i'd choose as ones right footed and ones left foooted)
It'd be the flying Dwarf, Shay Given in nets for me! I dont think there's another keeper in the game that consistently denies me 100% goal opportunities, he's outstanding!!
In this lineup you also got Good free kick and corner kick takers form each side (Chivu from the right and Gerrard from the left)
There's obviously flaws with this formation as there is with them all but i find i have a lot more success when playing this way.