Maybe some people have known this, but there is a class trait point rewarded for doing Ash & Star quest chain.

The quest has some chapters gated behind Osgiliath instances, but you can just run them at lv50 tier 1 if have trouble gathering a group. It may help you to understand the instance layout too.

You only need to complete two 3-man instances: Sunken Labyrinth and The Ruined City. I ran them with my lv100 hunter with little trouble, though it is recommended that fellow hunters should use Light Oil at the final boss of Ruined City to prevent damage reflection from the boss.