I've been playing against a Diddy recently, so far I've figured out that shines and F-smashes are your friend. The amount of range F-smash gives you allows you to play at a nice range from Diddy, it also allows you to punish a lot of his moves. Shine sort of speaks for itself, it comes out extremely fast - it's especially useful OOS. Diddy has a lot of cross up attacks when you shield, so it's the best way to punish it I've found.
Another thing you need to consider is how to edge-guard Diddy. He has an incredibley good recovery, but it's not impossible to counter. What I usually do is hold onto the ledge, then drop off and B-air -- depending on the situation of course -- sometimes I prefer just to D-air. If he's trying to sweet spot the ledge with Up-B, then just make sure to grab the ledge and press A at the right time. Just note however that Diddy can mix you up with his Side-B recovery and Up-B recovery, however if he's far enough out he's almost 100% going to Up-B. Side-B can actually grab you and throw you down, so watch our for that; Wolf's fast fall speed won't help you in that predicament.
I'm still studying this matchup, so I'm totally up for debate. I need to find out how to grab his bananas successfully, I'm pretty sure that's an important part of winning this matchup.