Marth makes it pretty nasty for predictable people with Counter. And assuming your opponents can't space well, you can Up-B pretty much anything that touches your shield. If they run out of invincibility frames and you're onstage, a Dsmash or Dtilt knocks them off. Charging a Shield Breaker or a smash puts pressure on people: Do I shield and risk getting poked/broken or Do I dodge and risk getting hit out of it?
Diddy can **** with bananas if you just chuck em around. If your opponent can't even DI then they don't stand a chance against Diddy's banana shenanigans, let alone the infinite lock he has with them. Speaking of no DI, Dsmash kills stupid early without DI and it puts them in a horrible position if they survive.
You might like to grab release them, Marth's pummel is fast enough to force a ground break and if they're not reacting as fast as they should, a quick Dsmash should remind them (Same with Diddy except maybe a banana toss)
If they like to spam ledge getup attacks then Up-B OoS with Marth will send them back off again, so will a banana with Diddy.
If they edgehog wrong and wear out their invincibility I think both Marth and Diddy can stagespike them with their Up-B's.
With bad players, you can usually get away with using some certain damage racking moves too much, or trying a bit too hard for the kill . Both Marth and Diddy aren't great at killing so this should help you out a lot.
Don't worry about DI, Nana's AI doesn't do that. If you can pull off Bthrow->Dair then Nana is pretty much gone.
