After completing the game, load your completed game save.
If you've been doing errands while playing, you may have noticed a man who constantly loses his diary. This man provides errands 79-85 from various locations around the world. When you accept errand 85, a new "secret forest" area opens south of Ding Dong Dell. This area is where you find the diary and meet the character who assigns you errand 131.
Completing errand 131 will net you this achievement, but there are several steps. Upon first receiving the errand, a new batch of bounty hunts will be available. Complete them all, redeem them, and return to the forest south of Ding Dong Dell. The process of "assign bounties, accept bounties, complete bounties, claim bounties, return to the forest" repeats four times (although on the last two you only get assigned one bounty each time).
After all of the errand and bounty running, return to the forest once more. After some dialog, the secret boss (The Guardian of Worlds) will now be accessible. Fight and defeat him for the trophy.
The fight itself is not terribly difficult assuming you have fairly high leveled characters and good familiars. You won't need an elaborate strategy either - simply defend against his "big" attacks (any time you see a text bubble naming an attack), attack whenever possible, and heal yourself when necessary. The only real threat involved is an attack called "Great Divide" which can deal 100+ damage even when defending. To counter this, just make sure to defend and watch your characters' HP bars.
For reference, this was my party:
Oliver (lv. 84) set to "Give It Your All" using Catastroceros (~lv. 20)
Esther (lv. 84) set to "Keep Us Healthy" using Underripe Buncher (~lv. 40) and Fluorongo (~lv. 80)
Swaine (lv. 83) set to "Provide Backup" using Bone Baron (~lv. 80) and Paleolith (~lv. 40)
Note: If you need to level quickly, I did so by farming Tokotokos outside of Perdida - it was fairly quick and painless.
Once you defeat the Guardian of Worlds, you can re-fight him at any time by returning to this location. He does become more difficult, but drops very rare items upon defeat.
If anything is unclear or could be improved, please let me know with a comment. :)