If there is a solution to beating Barcelona, or at least giving yourself a fair chance, it's to decide how you're going to go about defending and then commiting to it completely.
Either do it like Inter, Chelsea and Madrid (in the last few games) sit off, keep your shape and give up any hope of having decent spells of possession, or do it like we did at The Emirates and Valencia did earlier in the season and press high as a team. Both have risks, but at least you're giving yourself a shot.
Madrid, in the 5-0, and Utd the other night seemed to treat Barcelona as just another team, pressing in certain areas of the pitch, dropping off in others, and got torn apart.