While you most certainly will have this by the time you get the other trophies, you can easily grind this out early once you are around Level 30+. A good early grind spot is in the Zertinan Caverns. If you return to the room where you fight Adrammelech, you'll find that a very large amount of undead will continually spawn. They are easy to kill and killing them all will reward you with a large amount of LP. You can then leave the area and travel a few screens away and return to do it all again. With Golden Amulets, you will receive an even larger amount. This is also a good way to net some Gil and Exp.