They need to score two goals, that's where Liverpool can catch them if they will deploy Pennant and Bellamy on the wing, Kuyt in the centre and Gerrard in the top of the diamond... Sissoko has to discipline himself by not venturing too far above the circle line and Alonso must be wary of quick Barca break from Deco.. Liverpool can deal Barca a crunching blow!
Barca attack anyway 3-1 is always a bit awakward a scoreline to play but I think you should keep two backs of four and try to keep as much possesion as possible and if they score don't panic and stick to it. Although if they do score you lot know better than anyone how that can drive you on. I'd predict you'll get through although will be heartstopping at times, best of luck.