DO NOT TECH BEHIND GANON.
Well, not too often. You need to mix it up. Rolling away avoids everything but Wizard's Foot and a pre-planned Flame Choke. But do it too often and he'll just switch to using those. Going behind lets you avoid them, but will get you dsmashed if you are predictable with it. Best thing is to avoid it, but get hit with it twice and you can get a good idea of what they like to use (normally dtilt).
For heavies in general (well, the slow ones), I tend to play really cautious, allow them to close the gap a little before poking and retreating. After a little while, once they think they are safe coming at me, think I am playing defensive, I meet the approaches head-on. Avoid the hit then double jab cancel/dash attack/tipper dtilt or something into whatever. Heavies are combo food for Fox, and he eats them for breakfast. The fact that they are bad at approaching makes life even easier: You make them come to you, then punish them for it.
Then again, my technical skill is quite bad so I have to rely on game knowledge and mindgames like this... There's probably a better way.
Also, don't try to camp ROB. That gets you nothing but killed. You can rush him down more reliably than the others, I find. Just watch for sideB... It beats more than you'd expect lol. From the air is the way to go, just fastfall if he jumps to meet you. I need help with ROB too though, so pinch of salt.
re:DK idk I get my ass handed to me so hard by most of the ones I fight that I need to watch that one shofu vid to cheer myself up again.