This will probably be the last trophy you earn before the plat unless you put off grinding to max level because you'll need 6 playthroughs minimum to unlock all the bonus shop items.
First, you need to unlock arena mode and the bonus shop. That is done by beating the game once with every character on any difficulty. I'd advise easy or normal for this part. This step will also unlock nightmare difficulty, which is needed to do this in the lowest number of playthroughs.
Then once you have arena mode and the bonus shop unlocked, play with your favorite character until you have unlocked all the arena stages and the EX versions for Hugo, Yunica and Toal. The EX versions will show up once you've purchased Felghana Adol and once you have all three EX versions you'll have access to Napishtim Adol and you'll need to buy that at some point.
This leaves two more items to unlock and these are the ones that make this game need 6 playthroughs. So the next step is now to beat the game on nightmare difficulty with the EX version of Hugo, Toal and Yunica. You'll probably want to start with Hugo because he's the easiest to finish nightmare difficulty with since his EX version grants more range to his normal weapon. You'll probably want to save Toal for last because his run on nightmare is brutal and will require a lot of grinding unless you are really good at playing as him. I personally had to grind up to level 53 to beat the game on his path when I beat Hugo and Yunica's on levels 50 and 51 respectively.
Once you beat the game with everyone on nightmare, go to the bonus shop. You'll now see arena stage 7 available for purchase for 2 million SP. Once you have enough SP, go ahead and buy this and then you'll see Yunica's schoolgirl outfit available for 3 million SP. Once you buy that, this trophy will finally unlock.
Now, there is an alternative way to unlock this one, but it will bump you up to needing 10 playthroughs minimum for the plat. Arena stage 7 can also be unlocked by beating the game on easy, normal and hard with all characters. The plat also will require one nightmare run, so 10 playthroughs. If you go this route, just make sure you do at least one of those runs with each character with the EX version since that's the requirement for Yunica's schoolgirl outfit, but you have to unlock arena stage 7 before you can buy that. I'd only recommend this if you are truly finding it impossible to finish Toal's path on nightmare difficulty, but in most cases some grinding will resolve any difficulties you may have so I'd recommend to just do the three nightmare runs with the EX Versions and not go this route.