Okay so what I usually do is first I have Corrin and Jakob cover the right side of the barrier in the middle so they can deal with the oni, usually the two of them combined can easily take all the oni that come towards them then once Camila and Beruka come in I have them defend the left side of the barrier which they usually kill all the enemies that come near them as well. Only annoying part is having to deal with Ninjas debuffing her so occasionally I have Elise heal her up when her health gets too low when its safe and theres no ninjas.
On the top right I usually send over Arthur and Effie to take out the archers and I pair them so Effie can get Dual Guard and be able to 1 shot the archers. I also finally have Odin on the cannon dealing damage to Onis and the other enemies while having Azura sing to him. Finally I just have Nyx kill the two oni at the start of the level and then have her pick the money up at the house and come back.
Usually I keep Silas, Niles, and Selena in the middle as backup in case fliers get in and I need to deal with them.
It works okay up until Takumi activates the dragon vein. My biggest problem is that I'm not sure what to do with Nileas, Silas, and Selena aside from backup.
Your tactic is close to exactly what you want to do.
Except Niles should be on the arrow cannon on the right side to help lessen the load and help Arthur and Elfie tank out the right side. Pegasus come there more or less often.
You are a female corrin so this chapter is a lot easier than with male corrin. I assume you have Jakob as a great knight? If not that's okay.
Have those Silas and two in attack stance side by side, not paired up and go to the right side of the barricade and have corrin tank the hits while silas and jakob clean up.
Have Odin on the left side on the cannon with Nyx doing cleanup duty above them and Azura supporting from UNDER the cannon, not to the right. This lets you have more movement room to sing to Camilla or anyone else you need to if the magic cannon is fine.
On the left side with the cannon, what you want to do is use that to hit the lancers FIRST. Make sure the lancers are easy clean up by the time Beruka, Selena, and Camilla get there.
The oni on the left will take time to break through so you can ignore them until it is necessary to hit them, Since Nyx is standing on top of Odin, she can clean up the oni hella easy. Keep odin on that cannon and keep weakening everyone in sight. By the time the next wave of oni are there, odin should use the cannon to completely bring their HP down by half or more so Camilla/Belka can clean up.
Elise is whatever, if Jakob can heal let him heal. Just have Elise go to the left to get that 10,000 gold then keep her away from danger while healing when necessary.
If you do that well enough, it should be easy enough to clean up the right side with Camilla/Belka and then focusing on the middle right to keep Oboro and pegasus from breaking through.
You want to lessen the numbers so take advantage of attack stance for your tank character as much as possible. Use defense stance only when you feel too threatened or you are about to die. Defense stance is much more important later on but for this chapter, you want to use attack stance and have more units available to deal damage.
