Sorry for the late reply, I was at a family event, so I couldn't post.
Essentially it is:
-Have a mounted unit ferry your Staffbot down the left side of the map, said Staffbot has to have Pass to get through the enemies, so it can only work with units with access to Adventurer (ex. Nina, Niles, and Shura). Alternatively, you can also use the Chapter 24 Falcoknights if you had Niles capture them since they come with Pass and are mounted (though you'll want them to have a +1 move from pair-up).
-Ferry Azura down and she dances for the first staffbot.
-A second staffbot (has to be mounted with a Movement pair-up) goes down and Rescues Corrin.
-The first staffbot then Passes through the enemies and Rescues Corrin.
-Corrin goes up to the boss and hopefully processes at least one Dragon Fang to take them down.
You can see a couple examples
here and
here, though there are slightly different ways to pull it off (ex. I did it on the first turn rather then the second and used the Falcoknights rather then Shelter chains).
Since you're using Witch Corrin, some things I'll add is:
-If you have access to Rally Magic, Speed, Def, and Skill, feel free to use them on Corrin. Magic helps her damage output, Speed ensures she doubles if she misses the mark, Defense to ensure she survives a hit, and Skill to help that Dragon Fang process. Magic and Defense are easy to get through Izana (who I believe comes with it) and WL!Camilla, while Skill you can get from the Outlaws as Bow Knights. Also, tonics and whatever stats boosters you have left in those stats can also be thrown onto her, though they're not necessary.
-Since you have Warp, an alternate strategy you could use is to have a mounted Staffbot (or a mounted unit with a Staffbot) Rescue someone with Pass (ex. Shura as a Bow Knight), they walk through the enemies, wait, Corrin Warps in, and then goes up to the boss. I'm not 100% sure on this, but the strategy should still put her close enough that she can get to the boss.