Dark Ragne is a boss in PSO2.
It can found in almost any location in Emergency Encounters, multiple Urgent Quests, and in an unmissable story fight in Episode 1. It's worth noting that I only got the trophy for killing it in the Main Story quest. I fought it multiple times in Urgent Quests and in an Emergency Encounter, but it didn't unlock.
It's a pretty straightforward boss. It's capable of doing a large amount of damage so ranged attacks are the best way to fight it as opposed to fighting it in melee combat. Either way going for the head is recommended.
After defeating it the trophy will unlock!