Elaina was the first one to have her blade reach the Calamity Summoner, considering that her sword is much longer compared to Alfred's and Arthur's. But her slash did not wound the Calamity Summoner enough. It managed to cut off a bit of its skin, but not fatal enough.
Just as the Calamity Summoner gets far and recovers, Alfred follows up on the attack and makes the damage that Elaina did a lot deeper. The Calamity Summoner shrieks at the pain that the two successive attacks it received and is now in a panic. Prompting this, the Calamity Summoner decided in its one last attempt to defend itself was to summon every single insect and rat and block its way.
"I won't let you do that! HRAAAAAH!"
Arthur jumps in the air, his sword raised up high. Adelaide, who was watching from a distance, quickly cast an attack buff to Arthur without him knowing and let her twin brother do the final blow.