There are multiple defenses. You can attack their back because they are slow AF. They cannot fire without lowering their shield and exposing their head. So keep a bead on their head. No shield fully protects an operator. Blitz can be hit in the shoulder or legs. Montagne and Clash have full frontal protection but impact grenades behind them are deadly. Even in a corner you can shoot Montagne on the sides. You can melee Clash to get her shield to raise and shoot her in the right leg while she stumbles.