For this trophy, you have to defeat a rare monster until you obtain his crystal to use him as a monster, it took me 4 battles with this rare monster until he finally became mine.
First thing's first, the location of this monster is in Bresha Ruins 100 AF
, in order to unlock Bresha Ruins 100 AF, you have to go to "The Dying World
" and unlock the optional gate there with a Wild Artefact
. Once you're in Bresha Ruins 100 AF, you'll spawn where Atlas was in the regular Bresha Ruins in the beginning of the game, simply walk forward from this area until you get close to the point of an open walkway to the left, and a gate leading into the tunnel on the right, when looking at these, to the wall on the right, at the back of it is a piece of rubble. This little area here is where he will spawn.
To spawn a rare monster, you simply just keep making battles happen until they appear, a rare monster will appear outside of battle as a sort of paradox that you can initiate battle with. So you will know when you come across it. Take note that you can go into the Main Menu (Triangle) and go onto Settings and change the battle difficulty to Easy if you want to make the rare monster battle easier on you.
Now you know how to/where to get the Rare Monster, which is a "Tonberry
" (you'll remember them if you played FF13), keep defeating him over and over until you're able to obtain his monster crystal to make him your own, it took me a few hours to get him, and on my 4th victory against him I looted the crystal.If you're having trouble with the battle yourself, here's the tactic I used: -