A great area to do this is after the first antenna room. You will encounter suits of armor for the first time. While they are in combat with Trico, you can jump on their backs. Use the following buttons: Press Triangle to jump at their back, and hold Circle while you jump. You must keep holding Circle. Then, press the Left Analog-stick left and right for a few seconds. When the enemy lifts up one leg, let go of the Analog-stick. Wait a moment (until he puts his leg down), and again press the Analog-stick left and right. Repeat this for 30 seconds to get the "Get Off My Back!" trophy. If Trico knocks over the enemy, you must restart the checkpoint. Alternatively, you can look for a single suit of armor at later points in the story (when Trico is not with you). After the enemy grabs you, quickly run around and jump on its back. It is easier to do this during combat since the enemies are focusing on Trico.