I know a lot of you probably don't know me, or a lot of you just think I'm bad, but I've played against Felix's Diddy (I'm number 1 in the Pacific Northwest, he is number 2) a lot of times. And before you say he isn't as good as ADHD - That is probably true for the time being, but he gets better every tournament, and at Tourneyplay 3 he beat Havok 2-0 so clearly he has a lot of potential. So I am just going to say how I feel on the matchup.
I think the matchup first off, is 55 - 45 Metaknights favor.
Things you NEED to do to have success against Diddy:
Play extremely aggressive. A lot of diddys are now doing the "ADHD wall" with peanuts and bananas, they want you to camp and be behind in the match. You have to be smart while you're doing this though. What this means is RARELY, IF EVER, hitting diddy's shield. It is NO-GO if he has a banana, you're asking to get comboed if you attack his shield while he has a banana. What you can do, is attack the corner of his shield with dair at the top part, he cant punish that.
Attack diddys recovery from above. A lot of diddys have now learned that if they keep Metaknight parallel to himself, the over b has so much priority that it'll latch onto Metaknight, ruining your edge-guard. Attack his recovery from above-diagonally with dair or even fast falling fair.
Start the match off with dash attack!! every diddy either pulls a banana from the ground, or jumps and pulls a banana. I've gotten a quick starting gimp that way. If they don't pull out a banana to start for some reason, play aggressive as heck. If the dash attack hits, bthrow reverse fair and follow the edge-guard up. If they aren't used to the aggressive aerial b, you can do that after the fair and kill him, but most automatically air dodge, so wait for the read. If they jumped and the dash attack didn't hit, follow up with tornado or upair, diddy is useless if he's above you.
This is important for all matches, BUT DON'T GET DESPERATE FOR THE KILL! Diddy is probably the character who can punish kill-greed the most. If you start spamming down smash, diddy has one of the best shields in the game and it wont hit nor break his shield even if the shield is small! Just keep playing your game to get him off the ground. If you see the opening by all means do it, but do NOT get desperate for the kill.
Counterpick Brinstar if they ban Rainbow. Counterpick Rainbow, Frigate, or Castle Seige if they ban Brinstar. I've found success on Castle Seige by fighting for the first transformation, running and getting little hits in on the 2nd (try not to let him refresh his moves on the statues though, use them as bait) and running on the 3rd transformation. Be careful though, a lot of diddy's nowadays practice on Rainbow and Delfino a lot, which is why I didn't include Delfino (you can pick it I suppose if youre really comfortable there)
Shield up b diddy's bananas! When diddy is starting to throw bananas at you when you're both on the ground, if you're frame perfect your shield up b will hit him. This is a lot more suggestable at a higher percent, as he will recover quicker at lower percent and fight for control again. Try standing in front of diddy when he has 2 bananas ready and he'll likely glide toss one at you -- its kind of a mind game like an empty short hop or walking at them.
z-catch diddys bananas! when he throws a banana at you, short hop and press z as its about to hit you, and you'll catch the banana no lag! remember the same goes for diddy though, so be smart once you have the banana. Banana control is probably THE most important part of this matchup, so you can't give up the bananas back to him that quickly.
Be careful with your tilts! again, because diddys shield is soooo good, he can actually frame-perfect shield grab out of your tilt even without a banana if you're too close. Especially be careful for that if he has a banana -- you should just try to poke his shield at that point on the corners.
Keep diddy above you!! this is pretty common, but still extremely vital to racking percent. You can bait an air dodge then tornado, you can straight up tornado, up air, nair, whatever. Just try to keep him in the air AS LONG AS POSSIBLE to rack up damage.
I will come up with more later if people like this..