Hit the control stick away from your foe, then immediatly towards them and grab at the same time. If you did it right there should be significantly decreased ending lag on it. Very good way to grab someone approaching you.

Also, B-air spikes several times harder then D-air does and its not reliant on your opponent not DIing out of it.

PK Freeze as an edgeguard fails. It pops the opponent up very high and usually makes it easier to recover.... PK Thunder and B-air are far better.

With Diddy, Bananas are your friend. But! Only keep one out at a time and keep it close to you to prevent it being used against you. Also a very good edge tactic is to stick one just a foot away from the ledge. That way, if they try to stand up on the ledge they slip immediatly and take a hit. While if they try jumping over it they have your aerials to contend with.

When recovering, if someone comes out to meet you forward-b and rape their face. Then while on their face, hit jump. This causes Diddy to jump off them and spikes them downward. Its a weak spike, but it makes them think twice about meeting you head on again.

Popgun and Banana's stop approaches well. Up-B can spike if they're just underneath you when it fires. I don't play Diddy too often, but I know his dash attack combos extremely well. Dash attack to U-smash I see pretty often with tourny Diddys.