First, I'd be doubly fast. Sometimes, you see Mayweather fast with his hands during shadow boxing sessions but during actual fights he tends to slow down and focus more on punching power. I'd take advantage of that. I need to move in fast and release combination punches at the shortest time possible and then get out of his firing line and his reach fast. And then come back in at the right time.
Speed and Distance are Vital Here
Now that takes super footwork and punching speed. I need to close in to him rather than stay out of his reach all the time. Since he's taller than I am (I stand about as tall as Pacquiao is) and his reach is longer, I need to move in a lot. It's awkward for big guys to punch someone smaller who suddenly darts in close to them. It's more to their advantage if you engage for a while and them back off. That makes you an easier target.
Since boxing doesn't employ kicks and knee blows and take downs, I'd go charging low, watching out for his uppercuts, and then hit the kidney areas fast and hard. Once I hurt him there and he covers his lower body, that's when I suddenly hit his head.
You Get 5 Rounds at Most
Now, why did I say "the shortest possible time" in my first paragraph? Floyd Mayweather is a strong boxer and never been defeated by strong ones either. If I don't hurt him soon enough, he'd just easily exhaust me and let my own exhaustion defeat me. So, I've got to work out my strategies the first 5 rounds of the fight. Longer than that, I'm trapped. But since Floyd Mayweather is a strong guy and wouldn't succumb easily the first 5 rounds, I have to do my thing fast.
If Floyd Mayweather were just an ordinary professional boxer, you can try tiring him out if you're a strong runner like Manny Pacquiao is. Just dance around the ring evading and hitting him hard when he's tired and his guards are down. And since he looks too serious about this fight with Pacquiao, I can also put in a little Ali-style psywar to irritate him and destroy his calm and mental acuity. I've proven this effective during sparring. That is, if I were in Manny Pacquiao's shoes, which I don't think he'd do.
So, Would Pacman Win?
Who do I think would win? In my years of teaching martial arts and sparring with amateurs, all I can say is that Manny Pacquiao should be extra careful. He was knocked out by Marquez's lucky punch already (I don't remember any other professional boxer suffering like that in the ring) and it was due to carelessly putting down his center guard a bit.
Pacquiao would win if he hurts Mayweather terribly in the first few rounds and Mayweather loses his mettle and self-control because of it. He's so afraid of losing to Pacquiao that this could happen. But Mayweather would win if he proves as fast as he is during his shadow boxing sessions.