I don't have personal experience with this match up per se, but I play a lot of Ganondorf in PM and a lot of Roy as well. The one thing you need to keep in mind when you're dealing with a good Ganon that knows how to use his side b is that you are NOT safe in your shield. So your spacing needs to be ON point. The good thing is, if your spacing is on point, you basically get a free punish. Another thing to keep in mind is that if you think you can predict his down b, and your near the end of it's range (so he doesn't go behind you) you can shield it and then punish the MASSIVE ending lag on it. Watch how your opponent plays and get a feel for when he likes to use his down b. Once you can start reading that, you can even punish with your counter and DESTROY him for using it. You have to watch out for the side b mix up though. Also, remember it's NEVER a good idea to not tech the side b. Learn the tech timing for the side b (both grounded and air side b have different tech timings.) Ganon isn't really a hard match up for anybody, it's all about just knowing what tricks ganon likes to use and then making sure you avoid them and don't get get punished. Once you actually find an opening on him, his weight is perfect for being destroyed by roy's combo game. My advice is stay in his face as much as you can, make sure you spacing is on point, and when you're NOT in his face know what to expect from him (side b, down b, SHFFL fair/dair, SH side b, etc).
