Don't have much experience vs Dhd (at least good ones) but I can give you all some tips to fight campers in general. As a rule of thumb, but as has been mentioned already, raining fire on fragile ranged characters is good to go. But something which has yet to be mentioned is another, very important strategy: Distance and pressure.
I've had my fair share of experience on FG and fought quite a few Robins, Megamans and Links. They're different characters but the fundamentals for our ranged enemies remains roughly consistent. As mentioned, distance and pressure is key. Again, like my Mac post, I'll divide this as appropriate to make it more readable for all of ye.
********
Zigsta in another thread mentioned it's best to stay in their faces, which is true but you need to do so with a few caveats. You need to keep near the enemy but not so close that you're staring right into their face (Less of course you're punching/kicking them, then by all means). Keep roughly 1/4 of a screen away from them, keep your shield up, and wait for an approach. This is the distance portion but the pressure comes into it's own.
Another quick tip is, if you're not confident with using fire to engage (certainly vs experienced opponents it won't work that often to start but definitely throughout fights there will be chances), a running shield (raising shield mid run as projectile is about to hit you) approach can help close the gap rapidly. Be wary however of getting so close as to let the enemy grapple you.
One theme shared by most of the ranged characters is this: A crappy air game.
Once the enemy whiffs, either rack up the % with fire or straight rush them and grab/koopa claw. Once they're in the air, jump up but try to wait for them to either waste a fair or to air dodge. Then smack them. Once they hit the ground, run straight up to them, shield, and once they recover, assuming they have done so in place, up-b them back into the air. Koopa Klaw is also your best friend here as most will try to shield once they see you right at them, and klaw doesn't care for shields.
Again remember to space yourself well. Not too far so they can blitz you to pieces with projectiles, but not too close that they can get off a smash or a juggle. This is very general advice but hopefully it helps some of you dedicated Koopa mains.