Put Cavani as a advanced foward with attacking duty.Lavezzi and Pandev as inside fowards and Hamsik as advanced playmaker with attack duty.For me Cavani scores one goal per match in average.
for me,barca plays best if you put Pique as ball playing defender,messi and pedro as inside fowards and iniesta as advanced playmaker.mascherano as anchor.
althrough it is an good guide,next i will play with barcelona,that's for sure!