Okay, first off, Zapdos does not backscale from the 40-day storm feat. It literally is a third of the power necessary to fight Lugia, which is 3.82 Teratons of TNT. Meaning Dimitri has the upper hand in terms of AP.
Dimitri seems to have quite a few other advantages over Zapdos, particularly, he has more and better ranged options than Zapdos with his different magic techniques, particularly the lightning attacks that he has that deals massive damage to Zapdos. And the one ranged move that Zapdos does have, Ancient Power, is immediately nullified by Dimitri's Aegis and Distant Counter, so it doesn't even have that. This leaves Zapdos only the option to come in and physically fight., and which is a bad place to be to face off against Dimitri here. Not only does he have multiple abilities that can easily deal massive damage to Zapdos like Monster Piercer and Atrocity, but he has actually trained himself in the art of combat for many years, while Zapdos mostly relies on instinct in its battles.
However, Zapdos isn't completely helpless here. For starters, it's got a some decent stat boosting moves with Agility and Bulk Up, and any reduction to any stats it has will simply feed into Defiant, increasing its Attack all the more. Furthermore, it has a lot of defensive coverage as well. Detect helps it dodge perfectly, Quick Guard is literally a perfect move to deal with his spear if we go by Mystery Dungeon logic, and Light Screen can help with those nasty ranged moves that would be devastating to it. On top of this, he can use Counter, which would take one of those devastating attacks listed above and deal double the damage back if he can survive the attack.
Honestly, this is very close. Both are very heavy hitters that can deal a lot of damage to each other, and their techniques allow them to match blow for blow here. However, I believe Dimitri will end up having the upper hand for one reason. Unlike Zapdos, Dimitri has multiple ways of healing himself of any damage that's dealt to him. From his Vulneraries and Concoctions, to his Heal and Recover magic techniques. Like I said, this is extremely close, but I'm going to side with Dimitri for the win here.
Winner: Dimitri Alexandre Blaiddyd