As stated in the hunt conditions, Fafnir can only be found at Silverflow's End, during a raging snowstorm. Talking with Relj will trigger the condition, but the weather will always change if the party passes through the Frozen Brook. Instead, rent a chocobo, and ride to the south west (left, according to the map) from Mt. Bur-Omisace. No matter what you do, do not enter the Frozen Brook. Circle around toward Silverflow's End, then dispatch the chocobo and enter. If you do it correctly, Fafnir will be waiting.
Another way is to simply zone in and out of The Feywood until the Paramina Rift appears to be in a blizzard, and then go straight to Silverflow's End. The blizzard should not cease before the players arrives there.