Just space properly. It is a tough match because of his speed, but you have more priority. Remember that he can't kill all that well. His kill moves are Fsmash, Dsmash and some aerials (I think Fair and Bair).
Try to keep him off of you with projectiles. Overwhelm him with them. Do that until you figure out what can work against him. If he has bananas, try to pick them up with dash attack and use them against him. Don't focus on that though, cause that can be a distraction that could possibly cost you the match. I've seen some diddy's approach with over b as an attack and as the grab. The grab is used in case you shield, so be prepared to counter that with possibly a nair.
Their recovery is usually over b and then up b. Try to get him in the over b, but watch out in case he does that foot thing and if he grabs you, he can just diddy hump to gimp. Use your quick attacks on him, such as usmash, nair, and jab combos. IMO, don't use the usmash much.. otherwise you'll have a harder time killing him.