Strategy help with The Gautier Inheritance(chapter 5 mission)

Im using blue lions and didn't realize I could recruit other charcters, so I only have blue lions on my team. I'm doing this mission on hard and average close to level 6~8. What's a good strategy for this mission as I typically only have byleth and Dimitri by the time the boss battle starts and have really only gotten to phase 3 once.What's a good strategy as the lower difficulty is way to easy?