Offense
Flamethrower and retreating SH fair are very safe approach options against Dong. Dong has a hard time punishing Fair in particular. It's safe in shield if you fade back far enough. Nair of course confirms into jabs for a while since Dong is a heavy but it's low priority so be careful. Rock smash is also really good in this matchup because Dong is big enough to take all of the shards of rock as well as the main hit pretty easily (that's around 30% damage, yo). Rock smash also shield pokes Dong extremely well on block.
We combo Dong pretty well now at low percent. Dthrow to fair works extremely well and you might be able to catch Dongs DIing toward you on bthrow every once in a while as a result. Our strings out of dthrow can also go well beyond the guaranteed fair but you have to watch out for Dongs that mash out of the combo with nair. We also get somewhat high percentage (65 to 95ish without DI) combos with reverse Nair to tipper ftilt. Since Dong is a heavy, Utilt also combos into itself pretty well at low and mid percent. It's also possible to combo into Uair if the second utilt doesn't have quite enough range. Buffer the uair out of jump squat if you can though. Flamethrower is life in this matchup. Especially for very safe edge guards. Dtilt at the ledge forces Dong into a pretty predictable recovery at mid high to high percents which can set up a fair/dair kill. Dong also has no disjointed options so be sure you poke with dtilt in neutral. I'm not sure if it clanks with Dong's ftilt/dtilt though. Our bait and punish game is also very strong on landing Dongs since his landing options (ie. Nair) are pretty meh until you get into a position where he can bair you.
We have plenty of nice kill options in this matchup but Dong probably isn't going to die too early unless you Dair, gimp offstage, or get a hard read. Uthrow is a very nice failsafe as always. Also don't forget about our jab 2 options if you're having trouble killing at kill percent.
Defense
Dong has some really mean combos at low percents with utilt and uair and Dong can set this up pretty handily with a grab or dtilt (if it trips). Do not roll into utilt shenanigans at low percent. You will regret it. Rock smash and rock hurl can help us get out but if Dong baits that out you will eat a huge punish. Avoiding crazy strings at low percents will save you all kinds of headaches.
But the back breaker in this matchup has to be bair imo. It's quick and if Dong crosses up your shield with it, there's very little you can do about it unless you powershield. It can string into itself at mid percents too. Dong can also go very far offstage with his good horizontal recovery and pick up early kills with it. It's also great for mid percent stage spikes.
If the Dong you're playing is very adept with bair, you should seriously consider bringing flare blitz. Bair won't beat that heavy armor and the damage might force your opponent to respect your recovery more and put a stop to early bair kills off of the side.
Our landing options are pretty good this time around. Rock smash, as I've said, shield pokes very well and Dong doesn't have any particularly great options for punishing our Nair landings if we space and AC properly. Be ready to dtilt or jab when you land and you'll live unless he gets a dashing powershield. Be sure to mix things up to avoid that though.
Overall this matchup is pretty even. I think Zard generally racks up percent better than Dong from neutral but Dong has better early combo potential. Zards got the edge in neutral because his options are safer but that goes out the window if Zard eats a 40 percent combo early on. Zard has good kill options in this matchup but Zard has to avoid losing early stocks to bair. 55:45 Zard imo. Not sure what it would be against windkong.