Just found it, and from what I can tell, it could take an agonizing long while to appear.
The common tip I found about the enemy is to kill everything in 63F of Pharos, go to 64F and immediately return, as apparently it appears specifically in the chamber right before the stairs. But quite a few enemies are buried or out of sight due to how big the area is. Knowing the game enough just to do powerleveling, I'd imagine those enemies are competing with the Avenger in the spawn pool, so the first tip would be to go cycling as many times it takes to kill everything you find, including forcing the ones hidden to come out, and always looking at the minimap for red dots.
Another tip is to always move to 62F/64F as you come across their area transitions, and immediately go back. Since the Avenger has a 40% chance of appearing when you enter 63F, even if there are still enemies competing for spawn, imo it's better to have some chance of spawning it than none at all if you kept just cycling the area. And if you rotate between going to 62F and 64F, you still need to go through the chamber it appears in, so no risk of missing its spawn.
And if you're playing either of the Zodiac versions, or either of the earlier PS2 versions in an emulator, fast forward can help, as looking for the Avenger can get rather repetitive.
And a tip not in the title, if you're going mainly for the story and/or you're not overleveled/overprepared, but in either cases you started the Hunt Club sidequest already, if you see the Avenger, leave only 1~2 characters in your active party and make a run for the stairs behind it, swapping the characters as they die. The stairs lead to 64F, which has a save crystal, and the Avenger hits hard and can spam Ardor, which is AoE and can be pretty nasty on the characters' health, so better safe than sorry and with a broken controller.
Hopefully these tips help someone. =)
