Best Marth in Paraguay.

I also had a hard time against a Diddy in an offline tourney because of the little practice against them.
I went MK thinking he does well, not such a good idea, still won though.
The more air time you have, the better. Grab the bananas when you get the chance, but don't go chasing after them the whole match.
But if you use Fox, I don't see how you could win. Your best bet would be picking a good stage.

I'm new to Diddy, but what I do is face away from the edge, SH and pull out a banana, instead of dropping manually.
It gives me time to grab the ledge and prepare a spike. About 30% of the time I get to spike them after the banana flinches them, or only one of those happens.