The opponent might be hiding elsewhere, waiting for him to take the bait and then attack him with spells when he's unable to pull back, so Porthos kept a bit of power in reserve and was constantly cautious of attacks that might come from his surroundings.
But the musketeer never expected that he still guessed wrong.
The Magic Guided Conductor, seemingly indifferent, also pulled out a rapier from behind him, stepped forward, and crossed swords with him. Their swords clashed, sparking a trail of fire in the snowy wind.
And because Porthos kept a bit of strength in reserve, he suffered a great loss with the first strike—he clearly felt the opponent's strength was far less than his, and as long as he attacked with full force, the opponent would surely be pushed back.