This is a trophy that, along with "Is that....?", isn't required for 100% of the game and is therefore hard to find. In fact, without prior knowledge of how to find this, I doubt many people would find it randomly.
In the Lost Temple (see image below for its location on the map), go up, up, down, down, left, right, left, right. In this new room, approach the damaged forma on the right of the screen and he'll activate, unlocking the trophy. The forma will follow you slowly around the room but cannot be interacted with and cannot leave the area.
The directions you take to reach the forma is a reference to the Konami Code.
I recall reading a rumour that you had to find this forma before defeating the corrupted forma and getting the "good ending"; this doesn't seem to be the case, as this was the last trophy I got in the game, and it unlocked when I approached the forma.
The Lost Temple's location on the map:
The lost forma:
The lost forma's location on the map (the map still considers this the same location as the Lost Temple, despite all the screens you pass through):