Shuttleloop takes practice to get real good at. I think my personal best skill with MK (so I've been complimented even by strangers) is SL accuracy and tricks. The rest of my game is merely above average in most areas and being worked on, in my opinion. All this, and I still kill many heavies.
Against the heavier characters, remember they are tall. Things like full hop foward air are good options, like against a Grenade-holding Snake. You can get all 3 hits to connect to his upper chest/face, without blowing the nade on yourself, rather easily.
Meta's running grab is key here, too. Get good with it. You can grab these guys like crazy for an easy 11% down throw plus potential follow-ups.
For KO stick with down smash, neutral air and Shuttleloop. Tech chase forward smash is an option.
Any specific things/setups/etc you feel your friend DeDeDe is doing you can't beat?