Learn his banana tricks for yourself. Stuff like instant throwing, glide tossing, etc. In fact, I'd say secondary Diddy. Just for friendlies, but learn all of his tricks.
Uhm I don't know much about it, but it's my personal favorite matchup to play. Not becaue I'm good at it, but because it's just ****ing fun lol
Some little stuff I've learned though.
His shield will NEVER ****ING POKE EVER EVER EVER
EVER
****ING NEVER
Just a heads up on that.
Watch Dojo vs Gnes; he plays that matchup really well. And Anti and Shadow play it really well too, so watch their matches against ADHD.
If he dash attacks your shield, don't even bother trying to up-B or something. It has like no lag on shield (I think frame advantage on shield for him is like -4. So after a shield drop or a jump, he has frame advantage. And your up-B OoS won't hit him. It's stupid).
DI his dash attack. It really isn't that hard. A lot of people get hit by really stupid stuff like Dash attack > utilt string.
Tornado can be good, but you have to be careful with it. Like if you have one of his bananas, it might be harder for him to punish nados. And if he doesn't have any bananas, tornado is pretty much unpunishable. Also... spacing nado above his throw range is good. Like you can nado and space it so that the banana that he throws at you goes right under you, but you still hit him.
If he doesn't have any bananas, you can kinda pressure him with stuff. If you pressure him to the ledge and he doesn't have a banana, he doesn't have many options, and he's in a bad position.
If he has no bananas, Diddy's usually like to go for grabs, so that they can pull some more out. So watch out for that. Try really hard to punish his down-Bs. If you punish that, he's in a bad position, and you have control of a banana.
Oh and an annoying little trick that Diddy's like to do. If the Diddy is charging his popgun at you, there's a good chance that the moment you get near him, he'll shield cancel it > glide toss. It's stupid and annoying
If he has a banana in front of him and a banana in hand, don't bother approaching. He can cover so many options, and the risk to reward just isn't worth it. Wait for a banana to disappear then try and do something. And if you decide to approach him when he has a banana in hand and on the ground, probably running up and shielding to bait a glide toss > followup that you can up-B OoS.
Falling nair can gimp his recovery pretty well.
For stages:
You'll almost always want to strike and ban FD. If you can think of a reason for why you'd do better here than on SV, then ban SV instead, but I can't think of one...
If you go to SV, scrooging can help a lot. It's like FD, but with a platform that you can play gayer with.
Personally I kind of like BF in this matchup. I personally feel really comfortable on BF in this matchup, and with 5 neutrals with striking, you'll probably end up here.
Brinstar, RC, and Delfino are all good for CPs. And Norfair with MLG rules can be really good too, imo. Someone already mentioned Brinstar and the stuff you can do there against Diddy. It's Brinstar, just do Brinstary stuff against Diddy and you'll do okay.
RC is, again, RC. You're just all around really good here.
You can camp his ****ing *** off on Delfino.
On Norfair it's really hard for him to gain control of the stage, he can't really combo you, and you can pressure him with uairs and stuff.
It's a weird matchup that I'd reaaally like more experience in, but I find it really fun. Idk much about it, I'm gonna study some more matches first.