For those stuck on the First Riverbed Fragment Boss Fight and the Second Riverbed Fragment Boss Fight, here is how I beat it:
L = Lyra | T = Thorn | R = Rita
You will need the following skills for the 1st Boss Fight: L - Sacred Edge T - Last Breath (from Starting Item), Cover R - Shed Heart
You will need the following skills for the 2nd Boss Fight: L - Sacred Edge, Parry, Wide Open T - Last Breath (from Starting Item), Shock, Cover R - Impervious, Shed Heart, Rot Away (from Bygone Bale Item)
If you do not have these skills, go back to the very start (where you spawned) and there will be skeleton bones on the ground. Talk to it and fight it. When you beat it you'll get Eidoscope (to view the Gallery) and Purging Stone (to reset individual character's skills. Despite it showing "x1", as in only 1 use, it can be used multiple times).
The boss attacks seem to be hard coded (not random), so if you follow these specific steps you should kill it without any issues.