Celistia is extremely above people who one-shots people that are already above baseline, so the difference is not even 2x. Forcefields also help againts BB's strikes, and she can throw lightnings to paralyze him.
Ok, most of Big Boss regular stuff won't do basically anyhting to Celistia because she outranges him and most of his attacks will get blocked by her forcefields.
Invisibility is Big Boss best shot, however, Celistia is perfectly capable of dealing with Invisibles because there's even a regular type of Drag-Ride that can go invisible, meaning that invisibility isn't even rare of verse, so she can just nuke with Starlight Zero.
And one touch from Celistia's lightning and Big Boss will get paralyzed for dozens of seconds which would give Celistia a good amount of free hits.
First I want to know about Boss skill feats, I'm better answering at them rather than just giving them.
But for the most basic stuff, Celistia is only a Drag-Knight student and she is already considered the second most skilled Drag-Knight of the New Kingdom (number 1 is Lux), with all different kinds of veteran Drag-Knights with decades of experience looking up to her, and with the Four Nobles council saying none of them could hope to compare to Celistia.
If someone answers I will respond tomorrow, I'm going to sleep.