First of all, I don't believe there's one style or strategy that is best against Mayweather. We've seen him struggle against both Zab and Maidana who are about as different stylistically as you can be in boxing. Also, Mayweather is a great strategist, proven to be an excellent midfight adapter, who's been coach by world champions boxer/trainers since the age of 5. He'll adjust to any style thrown his way.
To beat Mayweather, it's not about the specific style you use against him, it's about fighting/boxing better than him, whichever style you choose, and obviously nobody has managed to do that yet.
So, pacquiao needs to find the best style for himself, of course tailored to this specific matchup. Mayweather does have "weaknesses", or certain aspects that aren't as amazing as his strengths: he can't produce a ton of volume, mostly due to issues with hand injuries and also doesn't have the best power in his punches. He also is very willing to back up, which can cause him to lose rounds in the judge's eyes, even if he lands slightly more punches. Also, the biggest mistake he made against maidana in the first fight was backing straight up too much rather than making liberal use of lateral movement. (He did correct this in the later rounds and in the second fight.) Against Zab, his biggest mistake was thinking too much, being cautious and allowing Zab to pick his shots and use his amazing hand speed (which back then matched Mayweather's elite handspeed) and Zab landed a lot of left crosses to the body.
His strengths are many, of course, but in terms of biggest advantage over Pacquiao, what stands out to me is infighting. Pacquiao needs to stay out of the pocket with mayweather, Pacquiao is not an elite infighter like mayweather is, nor is he a master of fighting in and and out of the clinch like mayweather is. If they get in close, I think mayweather will win that exchange almost every single time
I know mayweather's most obvious asset is handspeed, but one of the reasons Pacquiao is the best fighter PBF will have ever faced is because Pacquiao also has elite handspeed, so I don't think handspeed is going to be mayweather's guarantee to victory in this fight like it has been against so many other fighters.
Anyway, after that intro, this is is what I think could be pacquiao's best chance:
He needs to be aggressive but also very disciplined. Pacquiao is liable to leave himself open to counter shots, and mayweather is the best counterpuncher in boxing today. That's also how JMM managed to beat him. So Pacquiao needs to take advantage of mayweather's passivity and backing up to win points in the judge's eyes, but he cannot push it too much or mayweather will punish him heavily.
Pacquiao could throw 100 punches in the first round, but I don't think that will be a good idea, he needs to be in the 40-60 range, enough to please the judges but not so much as to open himself up too much.
He also needs to use the jab a lot to attempt to throw off mayweather's anticipation. Something like this, which Zab pulled off with limited success looks like a perfect tactic for pacquiao's abilities: http://stream1.gifsoup.com/view1/4414951/zab-judah-lands-on-mayweather-o.gif
Also, one of pacquiao's greatest improvements over the years is his right lead hook. He needs to try to throw over the top of mayweather's jab, and if he can eat a jab to trade a lead hook, that's a winning exhchange for pacman. Pacman needs to masterfully combine that right jab and lead hook. The dream for pacquiao is this: http://cdn.makeagif.com/media/3-07-2015/j2JPhR.gif
Also, with Roach's help, he needs to come up with new, possibly never before seen angles that not even mayweather sr will be able to prepare against. Ideally, this would be angles and punches that I, a random redditor cannot really predict, but I can speculate one example:
One punch i actually never saw before until recently is GGG's overhand lead punch, he used it beautifully in his last fight http://gfycat.com/ThornyAlienatedBird.
Maidana had some success with his overhand right against pbf, and if pacquiao can train to develop an overhand lead right, it should have more success even than maidana's non-lead overhand right. The overhand right attacks one of the few openings of the philly shell defense, and if pacquiao is successful with it, he would be able to force mayweather out of his philly shell into his more textbook high guard, a huge tactical victory if it happens.
Another slight clue to breaking the mayvinci code we can get from Maidana's second fight: Mayweather made a mistake trying to sneak in a reckless right straight at the end of the third round round. Roach and pac have to study the shit out mayweather fights to see if they can learn to time mayweather's attempts to sneak in a reckless punch as the bell sounds, especially in any round that's close, pac can steal it if he lands something like this: http://gfycat.com/ThornyAlienatedBird. Also, remember that mayweather basically gave away the next round to recover from the best shot maidana landed all night, if pacquiao does it perfectly, he could win two rounds with one punch like that.
Like Roach said, pacquiao needs to fight the perfect fight, and I think these are some of the things that we'll see if that happens.
