I did it with legitimate stats. You must be sure to make your characters’ speed an Hp as high as possible, specially Hermione’s since she’ll be essential. You’ll want all your characters to have at least one elixir with its 3 uses intact AND the recover staff, I managed to stay with 4 uses. Also: pray. Pray hard so you miss no hits against the dragon generals in your first turn because if you do, you’re fucked.
First step is to kill the 4 dragon generals. The mage dragon goes first, as he can’t attack in close range. A combination of Xerxes + dragon lance and Hestia + dragon Axe will do.
The dagon goes next, you can either go with Hermione + wyrmslayer or Isskrates + dragon lance, because they’re most likely to one hit KO with a crit. Hermione’s will surely crit the dagon and that is fundamental, if she doesn’t she’ll be severely hurt and Duma might kill her next.
As for the fire dragon, he’s the most dangerous of them all. You’ll have to use Fimbulvetr with Kisara (and her magic stat to the max, if possible) against him, then Paeon + Dragon axe. And it HAS to be in that order, because you will want Paeon at full force.
The ice dragon should be done easy. Crit him with Than + Luna, it shouldn’t be hard if you dedicated him time to maximize his luck and speed as much as I did. If you don’t, Tychon + Parthia should seal the deal.
And finally, it’s Duma’s time. If everything went as planned, he’ll immediately kill Xerxes and then Hestia, since they’re the easiest victims. Don’t panic. Do panic if Duma moves to attack them from a pillar, however, because then you’ll have to go to the northwestern corner of the map and kill the dragons there only so you can attract him to the center of the map (this is why you need the elixirs).
You’ll want to keep Than away to try and heavily hurt Duma with Luna, but you’ll have to be lucky to pull that off. Hestia died, yes, but Duma left the pillar to finish her, so now he’s at your mercy. You’ll have to surround him with Paeon, Hermione, Isskrates and Tychon, alll of them with antidragon weapons (wyrmslayer in Tychon’s case) and full HP. Again, be sure to keep Than and Kisara out of Duma’s range (should be easier now that he’s surrounded).
If your stats are like this, Duma won’t be able to double anyone. In fact, both Hermione and Tychon will double him:
I also had to kill those dragons on the left, but it wasn’t that difficult thanks to Duma enjoying Hestia’s blood. After that, just attack him with Hermione until you kill him, you should be able to make it thanks to the recover stuff and her great stats. In his following turns Duma will attack any of your characters (except for Than and Kisara) but as previously stated they’ll be fast and healthy enough to resist him. You’ll eventually kill him and rout all the remaining dragons.
I must say it was fun to create a strategy like this, it’s been some time since I found a challenge like this. The only other hard challenge I can remember is… Well, all of Fire Emblem Inheritance, but lets not count that game since it’s simply unfair.