This all happened in the 1 second the boss rabbit took to transfer the charge from its body to its horn. Sword, Shadow, Fire, and Agil, all of them determined the boss rabbit's target and made their move in that second with their incredible instincts.
Sunny already predicted that this will happen after watching them clear the rabbit groups before. There's no doubt Sword, Shadow, and Fire's had such instincts as they were capable of contending with an entire A-Class guild by themselves in the past.
And Agil as a Shield Warrior must have such an instinct to play at the forefront where unexpected changes occur from time to time. Sunny was really impressed by Fire's ability to find such a good tank.
According to his future knowledge, Agil had become Mysterious Fire Sword's no. 1 tank in the future and one of the important factors for the guild to become an A-Class Guild.