This is an automatic story trophy.
After the prologue, you'll be tasked in fighting four relatively easy bosses at the Fallen Sanctuary. Your abilities are extremely limited at this point so just focus on attacking them normally and healing when needed. Don't forget Virginia has the "mystic" ability that can heal your entire party with a heal berry, it'll come in handy with the all-targets attacks from the bosses. Also you can save in between the bosses with a gimel coin.