As the trophy description states, you must reach the summit with a companion. The summit can be found at the end of the final chapter, Sky.
All this requires is for you to meet a companion at any point in the game and complete the game together.
As soon as you both walk through the white light at the end of the final chapter Sky and watch the end credits this should pop.
**Note** This trophy can be known to glitch if at any time you or your partner are syncing your trophies before the end of the credits. To be safe I would just wait until the trophy pops before I checked/synced my trophies.
Hopefully this helps and have fun!