The leveling system in the game works with XP. Collect a certain amount of XP and you level up. You earn XP by completing Quests and hunting down machines. This trophy should come naturally when going for the other trophies.
If, however, you haven’t reached level 50 after collecting all the other trophies, then the quickest way to level up is to repeat the "Thunderjaw vs. Ravager Trial" located at the Sun Furrows Hunting Grounds over on the northwestern portion of the map. Completing this trial in under 2 minutes will earn you 10,000xp, and killing Thunderjaw afterwards will earn you an extra 3,000xp. See the below video from RaLph for a good strategy: