Rambling about Fairies - Manga Chapter 116
This sort of extreme 'to the death' sort of battle is foreign to members of Fairy Tail, but it definitely is incredibly familiar to Phantom Lord.
Fairy Tail fights all the time, but they never take it further than a few punches and bruises that leave angry wounds. Phantom has probably had cullings before, it's a dog eat dog world in that guild. The fact that she, meek Juvia that doesn't actively strive to be the strongest in any way, is an S-ranker in Phantom Lord, probably means that the guild itself is very competitive.
Juvia's calm response is a big juxtaposition to Cana's, and the reason is clear. She's used to this, and she's fully aware that tough, timely decisions like this one is something Fairy Tail members are not equipped to handle.
That's why she doesn't hesitate at all to sacrifice herself. It's no longer to save herself or stay alive, she's doing it because she doesn't expect nor want Cana to reach the same conclusion she did.
This is Juvia's own way of inheriting the spirit of Fairy Tail.