Wyrmslayer Trophy

Story Related. 

The Remnant in the Crater is Crater Ouroboros. Before you can go up against them, you'll need to clear the corruption in 3 areas (if you're having trouble finding them, hold  and an icon will appear marking their locations). Ouroboros will be pretty straight forward of a fight. There are 3 phases, all of which will start at the tail. When you're on top of the Remnant, you need to quickly move to each marker and destroy it. The markers with a circle on top of them can be grappled to with  whereas the ones without the circle can only be meleed. There's a timer at the bottom of the screen that tells you how much time you have to hit the next marker. If the time runs out, that phase will cancel out and you'll have to try it again. On the 3rd phase, the actual body of the Remnant will glow red. If you touch this, the phase will cancel out and you'll have to try again. If you die, you'll just restart the current phase you're on (the game autosaves after each phase). 

If you're having any trouble, please see VoidSprinter  for a video that includes all of the Remnant fights.