I just cant beat a good Diddy player. Hes just as strong as mario, but attacks crazy fast and those bananas are just evil when Diddy is dash attacking everywhere! I usually don't make threads like this but it just seems that Diddy is too perfect of a counter for Mario...help?
Keep to the air. Don't dash attack. EVER. Try to be on the offensive the few times your on the ground (you should focus on utilts or dsmashs if you're trying to kill). While in the air just build yourself a wall of priority with Mario's bair.
Your projectiles are pretty much the same, but Diddy's can charge, but that doesn't make much of a difference. Keep your spacing with fireballs, occasional capes, bairs and ftilts. Due to Diddy's crap recovery, you can edgeguard pretty safely with bairs. IMO the best way to take Diddy clear off the stage is to combo from utilt and chase off with uairs. Due to uairs fast start up and little lag, its possible to chain them, and since Diddy's recovery needs to be charged to be any good, you can just uair while he's charging up his jet pack.
Good caping is essential in this match. Not just for his recovery but for his fair and bairs. The timing is really difficult, but if you can predict when he'll do an aerial, then puts you in a good situation to chase Diddy off with uairs.
When on the defensive, don't even try shield grabbing (unless its a laggy attack, AKA smashes). DON'T EVEN THINKING OF SHIELD GRABBING HIS DASH ATTACK. It has so little lag that he's pretty much guaranteed to be safe whenever he dash attacks through you. A lot of Diddy's like to dash attack through your shield so as to avoid to get shield grabbed; a good way to counter this is to get used to jumping out of your shield. You'll be surprised how useful nairs and bairs out of your shield will help. If he's applying a ton of pressure, and you have a bit of space, bust out a nair from your shield, and most of the time it'll land and it'll give you some breathing room.