Win condition: Rout the enemy
You'll need Celica's Seraphim magic here – victory is likely impossible without it.
To start, move Valabar into enemy range, keeping the rest of your army safe. Once he's attacked by Necrodragons, have Celica use her Seraphim spell from behind Valabar at a safe range on one of the dragons. This should hit for half or more of the dragon's HP.
After that, place Genny behind Celica and use Invoke, summoning illusory soldiers to avert the Necrodragons' attention and keep Genny safe (and able to heal Celica if she's hit). Keep hitting with Seraphim using this strategy until both dragons are defeated.
Generally speaking the Necrodragons will prefer to attack illusory soldiers over your own units, but if they're afforded the chance to kill one of your units or take out an especially weak unit (Genny, one of your lesser mages, or Kamui come to mind) then they won't hesitate. Tread carefully.