Backhand slice down the line to Nadal's backhand every single time.
And by every single time I mean every single time Nadal directs a ball to Federer's backhand, which will be something like 75% of the time.
BH slices down the line will give rafa enough time to run around his forehand and hit a hard inside out FH which federer will barely get it back to set up a short reply for rafa to finish it off with his FH.
ANd if fed goes for DTL bh shots only (slice or otherwise) rest assured atleast 50% would be unforced errors.
Fed shd work his BH CC hitting hard with great angles and draw errors from rafa FH. Easier said than done but this is a very important shot. Mix it up and go for a winner when able to with the FH.
Federer knows all this. He knows what the game plan is against Rafa. Problem is Rafa forces fed to play low percentage tennis with his FH to protect his BH and errors happen. And Fed has to be timing really well and be at the top of his game to beat Rafa.
If I pick one shot its the serve. If fed can keeo his first serve % high enough and bring it on on the big points I think he will keep it very competitive.
I dont think a rafa win against fed on grass at Wimbledon center court is a foregone conclusion like so many people seem to think. Fed has to win the first set for sure if he needs to be in the hunt.