Bryialne

Tips & Advice
Running Decisive Battle with the Oath of Honor "Warrior's Challenge" will limit the map to only one random pre-boss area, which includes the area that Bryialne spawns in. You can immediately quit and restart if you don't get the correct one.

Additional Notes
In Decisive Battle, the Bryialne spawns in the second area. One of four floor variants exist, and he only spawns at the end of the one that begins with a 90 degree left turn into a short hall and then the giant pit.