In Fairy Tail Death Battle fanfiction, a character's reaction to the loss of a dear friend in battle can be very moving. Lucy might break down in tears, feeling both sadness and anger. She could then vow to become stronger to avenge her friend, which would show her growth and determination.
Another interesting reaction is when the villains in the fanfiction react to the Fairy Tail guild's unity. They might be initially confident in their victory, but when they see how the members of Fairy Tail support each other no matter what, they could start to feel a sense of unease or even fear. This would add depth to the story and show the power of friendship in the Fairy Tail world.