Like most bosses, use your surroundings to your advantage. With Commandant Steele-trap, it's best to use a Shotgun, SMG, Assault Rifle or Rocket Launcher and continually circle around her while shooting. It's pointless to hide behind anything since she is too fast to pick off from afar. Thankfully, the map is open wide so you can backtrack and move around freely. There are 2 areas you fight Steele. The trophy will unlock the second time you fight her (on the way to fighting the final boss going through the wayward pass).
Happy Hunting :)