Ni no Kuni: Wrath of the White Witch
34 Achievements
80-100h
King of the World
Awarded for defeating the Guardian of Worlds. Details for this achievement will be revealed once unlocked
7.7%
How to unlock the King of the World achievement in Ni no Kuni: Wrath of the White Witch - Definitive Guide
xTOTHORO
TrueTrophies
TrueTrophies
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. :)
Akari
This is the final quest, #131. You'll need to clear #121-128, report to the rabbit in the forest below Ding Dong Dell (the forest that doesn't open until after you beat the game once), then go back and accept #129 from the board, clear that, report back to the rabbit, accept #130, clear that, report back to the rabbit, then he will offer you #131. Walk through the door behind him to fight Guardian of the World.
If you're around level 80+, this shouldn't be too difficult of a fight. Guardian of the World has some decent AoEs, but the move you are most worried about is his laser. He will kneel and start charging, then fire a laser that sweeps across the field for large damage. You can either guard it, or try to time a spell with a time-stopping cutscene to make yourself invincible through it (I did this accidentally with Healing Hand). If you have Dinoceros, you can simply spam his rocks for large damage. Alternatively, since you can steal a full mp refill item from him with Swaine, you can equip Oliver with accessories to raise light damage, and then spam his strongest spells on him until he goes down. With 2 of the +light damage accessories (sold in Perdida), you can do 800-1000 damage per spell (but Dinoceros's Sling Stone was doing 600 for only 14 MP).
With my party at level 94, I killed him in 5 minutes on my first try with no effort.