This trophy is called "Of Legend and Myth
" - "Defeated the Ruler of Time.
This is an optional boss. To get to this boss, you need to have got to the portal in the last lands so you can activate all side quests. Then you have to complete every side quest. Finally you have to face every side quest boss again to make the this boss appear.
The side quest information is locating within their trophies:
Once you have done all the side quests, you should travel to the Laniff Ruins
Once there you will go in and teleport to another floor. Each boss is represented by a light than you can activate for the battle. You do not have to finish them all in 1 go. So you can defeat one, leave, save and come back in for the next. The light that represents each boss will respawn, but you can ignore them and run to one you need. The order you fight them does not matter.
The bosses are:
The first boss you reach is from Fides sidequest
If you go west at the first stairs then the next boss is from Kir's Sidequest
If you continue on the west path then the next boss from Nidr's sidequest
If you go east at the first stairs then the next boss is from Julienne's sidequest
If you continue on the east path then the next boss is from Endir's sidequest
The final side quest boss you reach near the top is the Time Judge which you fought as you reached the portal at the end of the Last Lands
Once you have defeated them all, the final boss appears. You will need Kir in your party with Dispel equipped. The boss will do a physical barrier and magic barrier spells which will stop all damage from those types of attacks. I found Battlecry from Endir and then Flare from Kir to be strong. The battlecry will increase the damage of Kir's Flar attack.
This trophy unlocks on boss defeat