Can't you join the goblins, then switch side back to the Tieflings at the Battle of the Circle Gate?
That is just another way of joining Tieflings actualy.
Steal the idol in the centre of the grove and all the druids should go hostile to you, but not the Tieflings unless you hit one of them with an aoe.
When i stealed the idol, back in patch 3, druids gone mad ... one of them screamed something like: "They gone too far now!"
And go kill Tieflings. O_o
No, but then the Tieflings leave and you can kill the druids without having to rescue Halsin and kill all the bosses.
I thought that one of premises is take the Groove for Tieflings ... so they dont need to leave.
Therefore, if you need to help them leave first, you kinda failed your objective. O_o
