To earn the "Blindsided" trophy in Turf Wars, you simply need to complete the DLC's opening mission. To get to the DLC, head into a previously saved game and head to the DLC tab on the menu.
The first mission is a pretty straight-forward gang clearance. The only difference from usual thugs is a new kind of jet-pack wielding and shield-bearing enemy using Sable tech. They don't go in the air; rather, they skate along the ground with the jet-pack and leave a trail of damaging energy. Best to save some Finishers to quickly dispatch these guys.
Once you have reached the bottom of the Sanitarium and enter the CCTV room a cut-scene will start. The trophy will unlock shortly afterward.