The huntress will have this as a potential mutation you can buy as soon as she join your party during the first mission after the ark. Simply use it on ten enemies, you can reload a save and use it over and over again on the same enemy if you want it to pop faster. I would say that your bound to get it when you have to take on a group which you can't divide and conquer, such as the crashed helicopter camp, where five enemies are standing close together and don't move around and you want this to keep the dogs at bay.