This trophy is awarded for defeating Slone, the final boss titan in the game. The fight is story related and cannot be missed.

Sloan uses the Ion class of titan, and has an extended core ability which can be devastating if you are caught without cover. Besides that though, she isn't that difficult to take down. 

She also calls in a number of other titans, in various classes, and Reapers during the fight. Prioritise these when they are around so that you have a clear shot at Slone.

The Legion class is recommended. Use cover and your shield ( - when it is available) to keep safe while you destroy the other titans with your devastating mini-gun. Once Slone has lost her lackeys, stay close to cover and pop out to fire off a magazine at her. If she uses her core ability, pop behind your cover to wait it out and reload. Hiding by the boxes on the top level of the arena is a good call. 

Slone will teleport out of the fight twice during the engagement after suffering enough damage. New titans and/or Reapers will be called in at these moments, so be ready to defend yourself. She will be vulnerable to being killed for good during the "round" where she calls in the Reapers, rather than assistant titans. Stay the course with your use of cover, recovery, reloading and attacking and she will go down relatively easily.