Fire Emblem Elibean Nights : How to survive and if possible, win in Raven's second mission?

FYI : Elibean Nights is a fan-made sequel to the Fire Emblem 7: Blazing Blade.
Here we get to play individual campaigns of a selected set of characters that were in FE7 (Eliwood, Hector, Lyndis, Raven, Merlinus… etc.)

I am struggling to find a way to win one of its missions. It’s the second mission in Raven’s Story : Undying Vengeance. Raven, Lucius, Pricilla, Geitz and Shen alongside one of the possible love interests of Pricilla (Heath, Erk, Sain) gets surrounded by a huge army of Etruria, led by General Arcard.

Initially, we get the objective of surviving for 14 turns. I’ve managed to do that, despite the heavy enemy attacks. Once I survive 14 turns, there’s a cut-scene and it gives me the option to Surrender or keep playing. If I select Surrender,

If I select No to Surrender, I get to keep playing. So far I have come to maximum of 19th turn but since I have limited resources and new waves of enemies keep showing up, my characters start to perish one by one. When all of them die,

So, how can I win this mission? Is there a different way than trying to survive? Or is there a certain level that a player have to survive in order to finally achieve victory?