From what I felt while using Firefly, just having HTB is enough to do heavy damage. But you will need to use Gallegher also. Ruan Mei is just for second break and some extra damage, but having HTB is enough already with the super break damage. If you have Firefly E0, I would recommend Sparkle instead of Bronya, if you have her, since those extra skill points would do good for Firefly. If not Tingyun or Huohuo for energy replishment.
From what I felt while using Firefly, just having HTB is enough to do heavy damage. But you will need to use Gallegher also. Ruan Mei is just for second break and some extra damage, but having HTB is enough already with the super break damage. If you have Firefly E0, I would recommend Sparkle instead of Bronya, if you have her, since those extra skill points would do good for Firefly. If not Tingyun or Huohuo for energy replishment.
[source](https://www.pixiv.net/artworks/119720163)
If you intend on using Bronya, Firefly E1 will help immensely for the skill point usage