This is it. Platinum trophy unlocked.
Just a few hints for some of the most obscure requirements of the last Group Battles in the Coliseum :
.Beat the game and save. Load your save and go talk to the old man in the Coliseum to access a special place.
.Beat the Cave of Trials (post game dungeon), defeat all the bosses including Gabriella Celesta.
.Learn the new musics with the Silver Trumpet. Save and play the final Demonic Aria theme on the world map. Exit the menu and
die prepare yourself.
.Beat the Iseria Queen (good flippin' luck).
.Go to the Coliseum to see a new Group Battle unlocked that is the most insanely hard thing to do in the whole game. Have fun !
WARNING : The actions described below will PERMANENTLY affect the final boss, giving all of his stats a x2 increase and the ability to cast & move at the same time. He also gets two new spells that will ruin your team, even at level 255 in Earth mode (easy).
Rest assured : In the remake, you can only do this AFTER you beat the game (you could do it by accident in the original and the PSP remaster, whoops !)
.Do Philia's Private's Action in Kurik as soon as you get to this town.
.Meet Philia again in Centropolis AFTER you beat the game. This unlocks the new version of the final boss.
.Beat the new final boss (gulps).
.Save and reload your save.
Now you can do one of the final Group Battle in the Coliseum which is a newer version of a previous one, but with the new version of the final boss.
Advice :
Always think to use a XP Card during boss battles or when you get into multiple battles, because it stacks to the Multi battle multiplier. If you maange to get the formation that gives a 125% XP boost at full power, it also stacks. You don't need to use a card at each segment of the multi battles, it counts for all of them, including some of the battles at the Coliseum that allow to use your own inventory.