This is for achieving an S Rank combo with the Fire Gauntlets, Ifrit, received during the middle of Mission 09, before your first encounter with Griffon.
There's a faithful and not so faithful way of getting this. The most consistent way of getting this legit is to do the regular YYYY combo against Nightmare's cores.
The unfaithful way to do this is to get a Stylish rank with Alastor, pause, change to Ifrit in the pause menu and resume gameplay, it'll unlock that way too.
It's something you'll definetly get along the way if you're going for a completion regardless, they're a necessary weapon for DMD.