For one, Fisk is fast and skilled. It'll be up to Kraven to land hits with poison weapons.
More importantly, Fisks's durability and endurance is through the roof. Maybe if you provided more specifics about how you think a poison is going to bring Fisk down, we could look at the specifics of how their showings compare.
Attacking his mind? As I showed, Fisk has a history of just willpowering through pheromones.
Durability and endurance? Fisk has at least two showings iirc of being shot multiple times and just... tanking it. Bullets in his skin and bloodying his suit but doing nothing to him.
😂 look at how many times Jessica Drew hits him, and... nothing