When Moku's voice was heard, participants throughout the arena stepped forward to attack their enemies.
In Claire's arena, as Moku expected, the orcs teamed up to defeat Boku first. Without any warning, they formed the Flying Swan Formation. When the geese's beak pecks, its wings will surround the prey.
One strategy against multiple enemies is to keep running and defeating them one by one. But in an arena with limited space, Boku knew he couldn't use that strategy.
If he advances he will be surrounded, if he retreats he will be surrounded. This is a function of the Flying Swan Formation, whatever tactics the enemy chooses, the result will be the same, ending with being surrounded.