To get this trophy, you must have the difficulty set to Expert while playing the game.
An easy way to do this (at least this is how I did it) is when you are ready to play the game a second time, change the difficulty to Expert and then Start a New Game. The new game will be set to Expert (Eligible for trophy). Alternately, you can set the difficulty anytime during the prologue and it will count.
Once set, you just play through the story. You do not have to be at 100%, you only have to complete the story missions only. Side quests, shard collecting, etc. are all optional, though it does make it a bit easier to be fully upgraded when completing the story as the enemies are a bit more difficult. Since I had played through the story once already and gotten used to the battles in the game, this wasn't really any more difficult than normal. Also, I found that playing through evil is easier as you don't have to bother with aiming that much, and can do a lot of head-shot kills to end battles quickly.
Once you change the difficulty, do not change it again until you complete the story. Basically as long as it doesn't say Expert (Ineligible for trophy), you will get the trophy upon completion of the story.