I don't think Diddy has as steep of an advantage as I projected a few months ago (I think I even went as far as calling it 7 : 3 in Diddy's favor for a week or so), but I do maintain that while the match-up is realistically closer to even, Diddy still has the slight edge.
As is the case with most heavyweights, their hurt-boxes are bigger and you can therefore rack up a great deal more damage at a quicker rate. If DeDeDe starts camping with Waddle Dees, power-shield them and walk or run past them, for the only reason they are even a minor nuisance is biggest they detract from your glide-tossing. Or, if you're fond of mild changeups, you can consider just short-hopping and throwing the banana since you'll get a good deal more horizontal range out of it.
My other rule thumb against DeDeDe is to never approach him without having already created an opening for yourself by tripping him. It may turn into a projectile-fest for a few seconds per stock, but you will have far greater success than DeDeDe in projectile pressure in the long run due to the sheer fact that you can manipulate the speed and trajectory of both your projectiles to better suit the circumstances. If you still get frustrated and feel compelled to approach, at least be armed with a banana peel.
As far as KOing D3 goes, I recall that you had some significant issues with killing characters due to an over-reliance on F- and D-smashing. Have you bothered to observe my earlier advice of literally
not using your KO moves period and just let your tilts and aerials do the heavy lifting? - I just went back and watched my first Loser's Semifinals set against Foursaken (D3) at OH SNAP! on Battlefield, and I knocked off his first stock with a completely fresh, unused D-smash close to the middle of the stage when he was at exactly 113%. This tactic requires a great deal of discipline, but you
will reap beautiful dividends later on.
If I had to recommend a smash attack for you to abuse, it would most definitely be U-smash. It has amazingly swift start-up, deals a solid 16% damage fresh, and has greater hit-box range in the rear. On top of that, the final hit of the attack has interesting properties all on its own: it deals the most damage, elevates Diddy high enough off the ground where most low attacks (
and grabs, including D3's) will actually whiff, and the horizontal hit-box in front of Diddy will actually
extend outward to a slight but noticeable degree. It has generally low knock-back and relatively short cool-down frames, which greatly expands the number of follow-up options you have out of it (U-tilt,
any aerial, etc.).
I would heavily encourage everyone to make U-smash, tilts, grabs, and aerial follow-ups your main sources for damage-stacking, and literally only use F- and D-smash strictly for KOing.
Lastly, platform camping. I highly enjoy playing D3 on stages with platforms, because once I get underneath him, I can camp him and follow his missed techs and subsequent rolls on the tiny platforms with glide-tosses all day long. His D-air might be an answer, but that requires him to at least short-hop off the platform or at least be falling towards it. In both scenarios, he is
still susceptible to eating banana peels and follow-ups. The other reason platforms are amazing is because they are low enough to where D3 will
still trip on them if they are on either platforms,
because he is so fat. This also includes Falco's chain-grab and Meta Knight's D-throw, for both these actions causes both characters hurt-boxes to extend upward.
EDIT: OH SNAP! v4.0 Loser's Semifinals: Foursaken (D3) vs. Le THieN (Diddy), Match #1