Normal parry TN, just like a ranged attack against someone in rearward.Indeed, the adversaries go after the heroes, then when the wolf jumps on the hobbit in rear-ward stance, by RAW, it's not the hobbit turn to play, then the hobbit can't change stance.
However, since the dynamic of the attack is determined by the stance, the question is how the rearward stance should work for the wolf close combat attack: should it be like a open-stance, so no bonus, nor malus die? Or like a forward one?