For what it's worth, in an actual tournament, any opponent worth their salt is going to autoban FD at the slightest possibility of going up against a Little Mac. For Glory is just a ridiculously broken mode, and Little Mac just happens to be one of the characters that really benefits from it. Unless Nintendo patches support for stages other than FD in Glory, that's just going to be the way it is (Battlefield would have been the best option, really).
That said, for dealing with Mac, it's pretty straight forward, albeit incredibly boring. Just hang around the edges, grab, toss him off, and gimp him. If you can get an early stock lead, all you need to do is wait out the clock near the edge of the stage. He has to approach you, and you're at a huge advantage there.
Even beyond that, against better Mac players, get used to trying to force an early stock lead and then waiting out the clock. No one really has the tools to approach him on FD, so he can easily control all of the momentum if you let him.
If you try to fight Mac on FD in pretty much any other fashion, you're probably going to have a bad time. So just wait him out. No one said fighting Mac has to be fun, just that it's doable.