I just hung out in the starting area storm and archered the hell out of them. I'm pretty sure everything in that map is flying, and you have at least three archers at this point.
Anyone manage to clear Birthright 12 without losing half their army?
Flyer+Corrin. Move to end point. Win.
You only need Corrin to escape, so its really simple if you just pair him with a flyer. If you want to clear everything, good luck. Xander is pretty much invincible unless you've been grinding like crazy.
Though you can get the treasure without fighting Xander or most of the enemies if you move fast enough. The outlaw standing next to it will swipe the treasure but he will still advance and attack you, so you just need to get in his range and murder him.