The defence's ONLY job is to convince the court that the prosecution's expert is incorrect in positively identifying the partial blood/water print on the ridged towelling bathmat as Sollecito's. They don't have to prove it's someone else's. If I were them, I would merely seek to discredit the whole pseudo-science employed in making this alleged identification, and perhaps point out in passing that the print matches Guede as much (or even more) than it matches Sollecito.
I think the defence dropped the ball majorly in this area in the first trial. After all, if the print was accepted as definitely Sollecito's, that's him pretty much convicted right there. The thing is, I don't think it IS Sollecito's, and I think that Rinaldi's "science" in this respect is bunk.
But again, it bears repeating that this appeal is a de novo trial. The defence therefore doesn't have to seek to "overturn" the first court's reasoning regarding the bath mat partial print. It merely has to convince a new jury of its invalidity as solid evidence against Sollecito.