Don't go too defensive. Settings like 'contain' and 'defensive' require perfection, and most of the time if you give Barca 90% possession, they'll eventually score. Since you're chasing the tie by 2 goals, but hopefully you scored 4 at the Nou Camp rather than them scoring 6 times at OT? So a 2 goal winning margin would be enough?
Try to use mobile CBs where possible. Vidic and Ferdinand are superb, but aren't that quick. I've had a lot of success with Djourou and Koscielny, personally but if you have more mobile CBs, use them.
Switch to 'counter' and go 4-3-3/4-2-3-1. Have a 'narrow' setting on your width slider (because they don't have anyone especially good in the air, so it forces them wide where they can't do anything) but have a neutral defensive line slider, maybe slightly deeper than 50% (so 35/40% ish) and use quick wide players yourself who are also good finishers (Walcott runs riot whenever I play them w Arsenal) so, Nani and Valencia? With Hernandez up top and then either play Rooney in the AMC slot (if going 4-2-3-1) or push him left and drop Nani if going 4-3-3. Have a ball-winner in midfield, who can also pass -- Carrick?
IF I were you, I'd try this:
VdS
Fabio/Rafael - MOBILE CB - MOBILE CB - Evra
Carrick - Anderson/Fletcher
Valencia - Rooney - Nani
Hernandez
With the instructions:
GK - Def.
FB - Auto.
CB - Def.
CB - Cover
FB - Auto
Ball-Winner - Def.
Box-to-box - Sup.
Defensive Winger - Attack/Winger - Sup.
Treq. - Attack / Adv. Playmaker - Sup.
Inside Fwd - Attack
Poacher - Attack.
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Make sure in your Opposition Instructions, you close down Dani Alves and the LB and show them onto their weaker foot, and close down Busquets. I personally don't think it works man-marking Messi, but you can do if you go 4-3-3 and choose a DM like Fletcher to man-mark him.