This can be done after Trico learns to catch you with his tail, which happens automatically during the story. He will do this when you are about to fall to your death. This results in a short slow motion sequence. A good farming spot to do this is shortly after the tunnel collpases in the mines, which is relatively late in the game. After that happens, you must climb out, kill some enemies, and go to a bridge. After a climbing section, there will be a cutscene. The bridge will collapse and Trico will fly to a tower. While he hangs from the tower, you can farm this by climbing on Trico's back and looking at his tail. Press X to fall down. You will see a slow motion sequence and he will catch you. Do this 10 times to get the "Lifeline" trophy. It only counts when there is a slow motion sequence during the fall. There are several other area like this throughout the game. Sometimes Trico will hang off a cliff or bridge. The same strategy applies, just fall off his back and he will catch you.