You mean to avoid bananas or...?
(I seriously know nothing about the Diddy matchup
![Frown :( :(](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
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With Diddy Kong, the ever presence of being on the ground is always the thing.
Flat Surface (FD) + Banana = Ownage
To break this equation, G&W must constantly be up in the the air, and almost never approach Diddy Kong with a Banana in his hand.
All Diddy Kong has to do is Shield Block you're basic and fundamental aerial attacks (B-Air, F-Air, D-Air), and Glide Toss a Banana out of the attack, which can lead to one his Smash attacks.
The pros of using Up-B constantly is...
1. You have the invisibiltiy frames
2. You're going vertically up and not approaching Diddy Kong
3. You're avoiding his Bananas
So in conclusion, just camp. Diddy Kong is a character you have to beat slowly, but surely.