Hack a turret with the specialist's haywire protocol and pick the option to gain control over it rather than disable it. To boost your chances of a successful hack you can increase your soldiers hacking stat with an upgraded gremlin, the hacking PCS, and equipping a skulljack. Once the turret is hacked it has 2 shots per turn to hit an enemy, so pepper an enemy with fire from your soldiers until its around 3-5 hp and then keep trying to hit it with your turret.