This Seeking Tech video guide will show you how to get the Immovable Object trophy and achievement in Crisis Core: Final Fantasy VII Reunion.
This will unlock when you do the following:
Defeated all enemies in Defense of the Junon Perimeter.
This event occurs in Chapter 6. Save your game beforehand in case any enemies get through the gate.
You can check out the video below to see how you can defeat all 30 enemies to unlock this trophy: