IMO, those are all easy matchups. Each of those matchups is fairly easy for FlyPhan, save for LuxChomp, which IMO isn't all that easy of a match against anything. But really, DialgaChomp isn't all that fast of a deck, and it thrives off of the face that it's got an automatic Snipe on Claydol, and how Dialga Lv. X can sit out and be an awesome tank. Flygon Lv. X just uses Extreme Attack on the Dialga Lv. X to shut down that route, and if you're stuck on Draw Power, Donphan can just stall out with no Dialga Lv. X to shut off it's -20 body.
Kingdra, yes, whoop de doo, it has weakness on Donphan, but donphan's -20 body with an Expert Belt can just neutralize this.. Plus there's the fact that Flygon doesn't have weakness to kingdra, and can just Power Swing for 2HKO's on the Kingdras w/ Expert Belts.
Cursegar- this is an autowin for FlyPhan. :\ What are they gonna do- use the SF Gengar to Poltergeist you to death? No. Any deck running Flygon and Claydol and a 2nd attacker and Techs won't have the trainers to let this happen. Now what, Shadow Room? Unown G on Claydol and you're good. Now, the actual Gengar of the deck- CURSE! OH MAN! with an Expert Belt, it can do a whole 60 damage to Donphan-70 With Curse. Then, it can TRAINER LOCK YOU! \o/ Again, FlyPhan PROBABLY doesn't have all the trainers for total lock down. Then, you can swing for 80 for one energy. :F Not all to bad for a speed set up, IMO, especially when it doesn't need Rare Candy to set up a Donphan Prime.
IF you want to test donphan against non-auto win decks try these:
More LuxChomp [yeah, this isn't an auto win for anything, this win is hard]
Jumpluff
Gyarados
and Mirror Matches