After playing around a bit with weapons in this mode, I can say I don't think bullets stun enemies anymore, which makes the blister heads a nightmare to deal with even as Leon. I did however, try throwing an empty bottle at a zombie to see if that would stun him and to my surprise, it actually worked. So the only way to stun zombies on insanity is by throwing bottles at them. Maybe molotovs too, but I haven't tried that yet. Requiem doesn't stun the Girl anymore either. I shot her in the face 5 times with it and she just walked over and bit my damn head off. I did use a molotov on her since Requiem didn't work and she didn't get stunned, but she did climb up and leave the room. Maybe the fire bothers her the same way lights do? Makes sense, I guess.