So I really wanted to share this, maybe someone will find it helpful! 

I've been struggling with Nocturnal Echoes beast boss, just until I boosted Lynn. My suffering was gone   (´ ∀ ` *)

So if you wanted Phantasio as badly as I wanted, Lynn is a way to go! 

Physce isn't a helper there solely because of the whole mechanics, once Beast enters her shield you NEED a strong mass attack to break her summoned weapons. I mean, if Physce is i6 she might be the deal, but at least on the basic level farming Lynn and wedges just more affordable. 

Same thing for Crystalophile, and even Psyche herself as a boss, for me Lynn is way more suitable and comfortable for these

As for the builds, you don't need much. Basically, you need crw wedges (I was buffing Psyche and gave Lynn the same wedges), for Lynn herself pyro-intented purple and that golden with pyro boost does the thing pretty well! This will be enough to just complete the boss and be happy (as I am with my beast man), and by that point you'll figure what you wanna do. Imo most important thing is to get Lynn i6, and then it'll be easier. 

Hopefully this will help!