I have been searching for GrimFeather for a while now, off and on. Mostly I stayed near the merchant camp, taking out hill giants and griffons. I read that Grim might spawn in the northeast near the wooden bridge so I was also clearing out mobs around there. But while I was doing that he popped up on track and was south west towards the spire. I know that sometimes a griffon will spawn there. No idea if the regular griffon is a placeholder or not.
GrimFeather dropped the rune of ivy on the first try for me. It took a few hours overall. Good luck to you, Ranger.