MC isn't really that surprised.And it make perfect sense for MC to be suprised. Since he wouldn't expect someone to fight so similar to him in this world.
More like he is exhausted because the opponent is a headache to counter, not to mention even his own partner going ballistic because of the taunts.
MC himself said that unlike his own Taunt Tactics which is based on information and planning, just for the sake of winning.
Vivian's Taunt Tactics seem just chaotic without any direction, and the taunts are being made just for the heck of it.