Need help with Rengokutsu Inferno

I am running Diting Fuqiu Dokidoki and Cachi the goat. Diting or Cachi dies when boss HP drops to~30%. Notably I don't have the Fortissimo shell on Fuqiu. Help me troubleshoot. Is it DPS or shell or build problem?

https://preview.redd.it/g782n14kd9sf1.jpg?width=2688&format=pjpg&auto=webp&s=3024b668fd04ebf2c38c981c47eeb685e48e9814

https://preview.redd.it/diptow5kd9sf1.jpg?width=2688&format=pjpg&auto=webp&s=66e15c1d78088077a66ecb483531f7b741295b6e

https://preview.redd.it/mjhgpy3kd9sf1.jpg?width=2688&format=pjpg&auto=webp&s=c98411d94fd7ceaf959815b80d4e8cb323b04916

https://preview.redd.it/d841mz3kd9sf1.jpg?width=2688&format=pjpg&auto=webp&s=c96770cbdc5510464b30a041b250521149252417

Diting 5-5-5 White fang

Fuqiu 1-1-1 Gagibyte

Dokiki 1-1-1 Artisan

Cachi the goat 1-1-5 Artisan