I don't like that usage of the word, "never." I've been taught to never say "never." Anyways, I want to hear more on the Diddy matchup seeing as you get to play Gnes on a regular basis.
Some essential Diddy Vs. Pit advice
1. Glide Tosses -- Pit has several glide toss combos using Diddy's own bananas. You always have to glide toward your opponent, but depending on the spacing, you throw the banana down or forward. If you throw it down but glide forward, you can dash attack backwards. If you throw it forward and glide forward, you can either Dsmash for a potential free kill, or Fsmash for damage. However, the weird glide motion makes the fsmash automatically DIable after 100% because of the sliding.
2. How to play against Diddy whilst being at low percents (0-50) -- At low percents, you need to make sure you stay far away from him. Diddy capitalizes far more on opponents at low percents than any other time. Since this is the case, the most viable strategy here is to either stay near the ledge or on platforms, but be weary of any traps he may have set up. Spam arrows here to aggravate the Diddy and force a more aggressive playstyle. If you have the lead, he has to approach. Basically, build the lead here.
3. How to play against Diddy whilst being at mid percents (50-100) -- This is where a little of the ground game mixes in. You're going to have to change your playstyle completely. Instead of running away and always being in the safest spot, (ledge/air) space dtilts/fairs or airdodge inside of him and utilt (MOST EFFECTIVE). Side note -- Do NOT forget to abuse utilt. It balances over Diddy's banana throws, it's 2 frames, AND it does 16%. -- This is basically just rotating your playstyle to not be as predictable, and mixing up your campy playstyle, so that he can't adapt fully to your camping. So at mid percents, start racking more damage with tilts and smartly spaced aerials to divert your opponent.
4. How to play against Diddy whilst being at high percents (100+) -- This is the most dangerous part of the match. The only time you should be here is when he's at low percent. A very risky part of being this high is that any banana is a potential kill. The match started with you playing campy, then aggressive, so the Diddy player will be sitting in his shield alot more due to your aggro style. Grab here. Always grab and throw the player away, then spam arrows. Remember you have 2 reflectors during this match, you can use them different times, but make sure to do so sparingly. This will aggravate the player, as this style is annoying to fight against, but effective. Capitalize with charged smashes and pivot grabs as well. So at high percents, capitalize on the shielded opponent and go back to safely spammed arrows in between throws, with CHARGED smashes for mix-ups.
Also, something I'll touch briefly on is Jump-Canceled throws. Pit can naner lock Diddy just like Diddy can naner lock Pit. Just JC throw instead, it's the same length as Diddy's glide toss.
I just wrote a little bit, but I have to go. If someone has questions/comments, let me know. I wasn't too clear but I can explain things alot better 1 v 1. Let me know guys. =P