Finally cleared the Fell Xenologue on Maddening at Chapter 18
Just wanted to celebrate and show my team just as another resource for anyone looking for ideas to clear this themselves. Some of the posts here helped my decide in my team.
I went through the middle island up to the top right relying on Gregory to use heals to refresh his goddess dance as often as possible. I ended up killing all the bosses myself except for Timerra who I lured to the first island on turn 2 with Astra storm so I didn't have to deal with her later in the map.
Ivy/Soren was my main enemy phase unit, she could reliably kill wyverns and most other enemies fairly safely with her healing. Celine with Yunaka's dragon vein was very helpful to get kills without worrying about counter attacks. Zelestia even pulled her weight by being able to safely kill the corrupted wyverns in the final stage of the map. Kagetsu was my main damage dealer against Nil.
Ivy/Soren and Celine/Chrom were hands down my MVPs for the map and the whole Fell Xenologue.
I did end up losing 2 units; Madeline and Zelestia. Madeline died while being a wall during the transition between the middle island and the top right. Zelestia died from a sandstorm proc against Nil. I also had 5 rewinds left by the end.