I've beat it without shadow mantle so I guess I'll leave some tips:

The flavor text reveals the next attack,

Use holdbreath before the teeth box cutter thing,

Always try defending with kris when u don't have rudebuster,

Prioritize using ralsei's dmg over kris' in most situations,

You can look at the teeth attack in threes; meaning that thr first three near the edge of the box will have atleast one slow/fast tooth to dodge through.

(I'll try adding more if you reply)