I really feel that this is strongly in DK's advantage.
I'm sort of biased because the very first DK main that I fought was axelol, and I've held every DK I've fought since then up to his standard.
Maybe it's just me, but I think it's best to approach DK from the front. On the ground, you can handily shield his (massively long-range) tilts, smashes, etc, while dodging his grabs. It's not always easy, but it beats being behind him and getting U-tilt juggled and B-aired.
In the air, the only quick moves he has that can hit you are N-air and B-air (he can, like Sonic, hit from the front), but his speed (and range) is much slower in the front than what you'd have to deal with behind him. Keep note that with proper prediction, a DK CAN land F-air and D-air (even... on spring jumps) if you're approaching from too far. When you're up close and he's in the air, feel free to attack from the front. If your opponent is familiar with the Sonic matchup, don't expect to just spring and use an aerial to get by when you're near the edge - you'll end up getting spiked.
When approaching from the front, just watch out for:
- 10-charged punches, since super armor will counter and destroy you. Also keep in mind if he charges his punch but isn't flashing. It's most likely a 9-charge punch - no super armor, but ungodly knockback.
- Up-B. It also has super armor (or is it invincibility frames? I think it changes depending on grounded/aerial up-B) during the beginning, so it can be used to counter and rack up some un-wanted damage. It IS possible to smash him or grab him out of it, but it's usually safer to wait until post-attack lag.
- Sakurai Combo. 'Nuff said. Too many times have I gone in, expecting DK to tilt or something so I'd tap shield, and as soon as I let go, get slammed into the ground...
But really, B-air/headbutt hurts your shield pretty badly too.
- Ground Slam - clangs with/outprioritizes SDR's.
- Grab. This is where you'll need 1337 teching skills. at 40%+, DK can cargo-grab you, walk off the edge, and stagespike you with a throw. Good luck on this one ;__;. Ripple also posted a video of what to do to people who get caught doing reflexive teching (throw them under the stage/away from it and lol as they airdodge). However, Sonic is rather safe from this since he has such a large recovery range.
What's interesting about this matchup is that DK's not very disjointed in some of his attacks. I'm not sure exactly which moves, but you can grab/attack him from his extended limb range. An example that I've brought up a few times is that I've stood halfway in Brawl Yoshi's Island as an opponent DK did a ledgehopped DK Pawnch and I tipped his extended hand with my own, either at the same time or immediately after he missed, and it hit him for the kill.
Like Dedede, rising/lagless B-airs can totally screw up your approach game. It's also one of DK's best approaches/spacing tools, and can even KO. A running P-shield can sometimes screw up his spacing if he's doing a falling B-air, and you can proceed to grab him out of it. It's much easier to do this on laggy landings, but I'm pretty sure you can grab him straight out of the aerial if you shield the attackbox.
okay, i'm tired. have fun with that.