To defeat this master elite remove all roots ans specially bees, since they heal Grimbark, and do that first before focusing on the tree again. That will save lots of distraction and missing shots.

Do damage over time FM is its possible (Yellows).

Hunters can do a good use to the Rain of Arrows skill.

Roots will attack (if present) minstrels. Keep aggro from them by simply killing the bees.

A great example of a quest that needs good guardians or captains to gett aggro while the rest of the team kills adds and try to keep the group alive.