When Unno said this, I first thought he meant that Gai was too strong for Nezu to defeat. But then Kakei expresses his surprise at Gai still being powerful enough after turning human, which means the Braves didn’t know about his newfound strength beforehand.
And that’s when I realized Unno was saying Nezu must have found it hard to fight Gai because the boy was his student. Nezu did seem regretful at having to face his pupil and even told him he’d only go all out once Gai proved to him he could hold his own. He probably formed an attachment to Gai after being his teacher practically the prince’s whole life. I mean, he was still babysitting the kid during the invasion lol.
This kinda makes me lean on the Braves all caring for Gai’s wellbeing as an individual, not just Ramaga’s future host. They know it’s their duty to serve Ramaga, so they can’t disobey orders, but they do their best to raise him as their own until the time comes. Even now they still refer to him as “waka” and don’t specifically blame him for Nezu’s death. I feel sad now ya’ll 🙁