The Last Guardian – Get Off My Back! Trophy Guide – Cling onto a suit of armour’s back for 30 seconds or longer. [Silver]
A great spot for this is after the first antenna room. Here you’ll encounter suits of armor for the very 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. Hold Circle-Button while you jump! You must keep holding Circle now.
Now take the left stick and push it left and right for a few seconds. When the enemy lifts up one leg you must let go of the stick. Wait a moment (until he puts his leg down) and again push the stick left/right. Repeat this for 30 seconds.
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 isn’t with you). After it grabs you quickly run around and jump on its back. I found it easier to do in combat cause the enemies will be focusing on Trico and aren’t moving around so much.