A Dark Ragne can be found in the tutorial but for those past it there's still other ways for this encounter.
Time Attack Quest Running Maneuvers: Naverius I
Two for one special here. You can encounter a Fang Banser in the Tundra area
The paths vary. When you arrive at Area 3 take the West Exit which leads to Area 4. The boss will be waiting.
The quest ends with a Dark Ragne
This is a Single Party quest (4 players Max)
Starting Difficulty is Normal 20 & Up
There are no level restrictions.
Alternatively, progressing through Episode 1 of the Story you'll be forced to fight one while you're eradicating the Falspawn from the City. The Quest is called Falspawn Full Assault
The unlock requirement for Naverius I are
1. Clear "Floating Continent Exploration" quest (any difficulty)
2. Complete "Difficulty Level Trial II" client order from Cofy. This client order appears when you hit level 35 with any class.
Thanks to AI Source for confirming the unlock conditions