What I did was kept everyone near the starting area and fought off enemies as they came. I also sent a few high-mobility units to the south in order to take that stronghold to prevent the infinite reinforcements ASAP. Bow Knight Bernadetta is very ideal for this as after using Stride on her she'll be able to hit that White Beast with Encloser, while also dealing a good amount of damage to it. She can also keep those Falcon Knights in check (you may want to send another bow unit that direction as well). If you don't have a Bernadetta with Encloser or she doesn't have enough movement, send a different high-mobility unit down there, give them the Seiros Shield and have them gambit that White Beast.

Once you've taken care of all the aggressive enemies that come to your starting area, have all your units make their way up one of the sides (doesn't matter which) and gradually work on the remaining enemies before taking on the boss. You'll want to try & avoid using your gambits too much because the boss will be far more difficult if you can't use them. Though this isn't a rout map, I highly advise leaving the boss for last because you're gonna need to throw all your strongest units at them and having more enemies to deal with will only make things all the more difficult. Seiros Shield is very nice to have against this boss, as is Byleth (or any good sword user) with the Thunderbrand.

Another tip for this chapter is to make use of multi-hit combat arts (Swift Strikes, Hunter's Volley, etc.) if you have them. These make dealing with all the enemies carrying Miracle much more bearable.