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Disclaimer: I'm garbage at doing MU write-ups.
Not a lot of practice, and my thoughts go a little bit everywhere.
I type how I think. I'm getting better tho.
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I think I've played enough Diddys at this point to have a say:
Ratio: 55:45 Diddy's favor.
We won't win the forward air war, or the mobility war. But we don't have to.
Like any player and their main, they all come with personalities. You're going to have the grab hungry diddys, the fair spamming diddys, the I-can't-be-anywhere-without-my-banana diddys, and you'll have those diddys who will be well balanced in all of these areas.
There have been some days where I can't fathom beating diddy.
https://twitter.com/MisterbeepEric/status/563523098742652928
And I hope a lot of us aren't sinking into that mindset like I have before. Let's save that mindset for the Sheik MU

I kid, Sheik isn't that bad. Just harder than this one imo.
So what do we have going for us:
Nair
Nair can be used in this MU just as it can in most of our other MUs, except that we should be wary of a few things. First, if diddy challenges our falling nair with an uair, the diddy will most likely win unless you space it so well that he whiffs. Perhaps he just messed up honestly. What we can do is bait out diddy attempting to punish us by either hovering in our up b at a safe distance or even by not trying to be offensive with a fast falling nair and just space to miss him. And if he tries to run in and cover you, you'll most likely win the exchange. Just be careful when trying to recover back to the stage with nair. If diddy doesn't realize he can challenge with u air, then have at it. Do what you do. Land > jab, land > roll, land > grab. It's basic MU stuff from there. Just see what the player is doing and react accordingly.
Nair can also be used to zone. This one's my fave. With all the gyro love that we be giving him, why not spam just as many SH nairs? If he rolls, you retreat with the nair and tag him with it. If he's behind you when you hit him, turn around and get a sh fair follow up. After the fair, hang back a little. Perhaps land and go into shield as he might land with u air. Or space away after the fair and gyro his landing. It's all you =] This applies to a lot of MUs btw. And you know Diddy's are thirsty for the roll > grab option. This works well with the grab hungry diddys.
What if we nair his up b? If we are a ways above him while he's recovering, it's our best option to contest with his recovery. Odds are, we stage spike him.
Gyro
Need I say more? Maybe not but I'm gonna.
Gyro that monkey flip.
Gyro that grounded banana.
Gyro that forward air spamming. Oh his fair cancelled it out? How about another.
Turnaround gyro into the stage as he recovers. Gyro can bounce off the stage and hit his recovery.
B-rev gyro cancel to mix up recovery bc you know we don't like being uaired.
Gyro's a beautiful thing. Use it and abuse it.
Uair
Yo we got one too.
We can stuff those high rising side bs all day if we see them coming.
Ever landed with uair before? I love to. It either leads to an utilt, a fair, a dash attack. It all depends on where they pop out at. But diddy is at a height where we can get away with it. Trying throwing a few in here and there. Don't do it in tourney without getting used to it tho.
Laser
Run away grounded b-rev laser can combat diddy side b
Laser him as he goes for his banana (whether when he's jumping to z catch it, or when he's trying to pick it up. Tho I like gyroing him when he's trying to get it off the ground).
Diddy do's:
Keep a gyro out as much as possible. Diddy loves to stay mobile. It's in your best interest to disrupt that. Now don't do this mindlessly, as a gyro can contest with side b, add coverage as you're landing to the ground and can be a pest to his banana game.
Stay mobile yourself. No we aren't as mobile as Diddy, but don't play scared. A scared ROB is just begging for standard grabs and command grabs. Also, don't just move to move. Move to stay evasive, to remain hard to read. Don't get ahead of yourself and waste your dbl jump just hopping around.
Play Diddy some yourself! Learn how he moves, the distances he travels (for example, diddy has not only 2 side b variations, but also 2 distances he can travel out of side b)
Diddy don'ts:
Don't stay still. Explained in the do's section.
Don't jump mindlessly at him when he has a banana. Don't just nair in place, expecting a roll. Or at him, trying to make him shield drop. He won't. You will get tripped.
If you are at the ledge, be very mindful of your options here. Especially if the diddy has a banana. You will most definitely eat an f smash if you descide to get up, get up attack, or roll from the ledge while the diddy has a banana planted in front of him or in his hand.
Don't underestimate Diddy's fsmash or forward air. Fair is super safe. Diddy's fsmash second swing reaches a decent distance further than his first. Don't get caught slippin (harhar)
Don't be fooled that a diddy will just empty land when he is close to touching the ground but has yet to toss out an aerial. His u air would love to have a word with you.
Also, the diddy can mix you up and empty land, then follow up with a grab. Have a game plan each and every time.
Don't be fooled by side b lag. A lot of the time the diddy will do it high enough to where he can throw out an aerial is you run in to punish. Try a gyro, try a powershield. Test it out.
Don't miss your up air! If you do, you will eat one and most likely die at certain %s. Not fun =[. Bait air dodges out of down throw if you're uncertain. Or if he has a habit of jumping out of the downthrow to u air string, then you should still be able to connect if you choose not to bait since everyone's airdodge comes out faster than their jump. Play around with those scenarios.
If you are standing still and a diddy is side b'ing at you and you're not rdy to gyro him, or he's too far off the ground, you can either:
A.) Short hop air dodge in place > land > grab
B.) Roll into diddy, which will turn you around, then gyro. (I haven't fully tested this yet. But it works in my head.)
Following a lot of these don'ts well help you keep up with diddy's pace. Most ppl do more stuff wrong than right in this MU atm. That's why I have more don'ts than do's.
Hope this helps <3