This trophy is for completing the first chapter of the game. Being the first chapter, you don't really need to grind that much.
The final boss of the chapter is Gogmagog, who you'll fight twice. Both battles are identical, however he is ever so slightly stronger in the second encounter. The best way to fight him is a combination of the RAV/RAV, COM/COM, and SEN/SEN paradigms. Hit him once with COM/COM once, then switch to RAV/RAV to build up his stagger gauge. Once it's empty switch to COM/COM to deal more damage. If he's about to swipe you can switch to SEN/SEN to reduce damage. He doesn't do very much damage regardless so you might not need to to switch to SEN/SEN. If for whatever reason you end up at low health, feel free to pop some potions. After beating him the second time, you can examine the large gate to end Chapter One.