Actually, this battle is quite easy to explain. Team D played the role of disruptors hiding inside, casually firing magic bullets to throw the enemy formation into chaos, and then Team A and Team B sneaked in to attack the others.
However, Team B got lucky. They found Grayfia, stunned her with electricity, and brought her out.
"To be honest, the place you chose is so good that it just lets us fight guerrilla warfare."
Chapter 218