You can change target when shooting with right/left shoulder button, that works fine. However, it does not work properly if you try to use other skills like bash or mark for death. You still see the RB/LB prompt and list of valid targets but pressing the shoulder buttons takes you to next trooper instead.
The only way to select another target is to scroll with the joystick to choose target on map. However this does not work on mindfraggers as they overlap the target so there is no way to bash mindfragger without using a mouse.