Broken Isles Pathfinder, Part Two is completely unlocked in patch 7.2. Compared to the previous content, it is much simple.
Unlocking Legion Flying includes 4 parts as follows:
1. Broken Isles Pathfinder, Part One
The first step of Unlocking Flying is open at the beginning of the Patch 7.0. We believe that most people have completed this achievement. The difficulty lies in Suramar storylines which need Exalted with The Nightfallen.
2. Explore Broken Shore
Because the Broken Shore is small, it is easy to complete Shore world quests.
3. Defender of the Broken Isles
It is related to the Legion invasions in the Broken Isles zones. It involves stopping an invasion in each zone. This scene quest is very simple, so it is easy to complete.
4. Legionfall Commander
Armies of Legionfall is a new reputation of the Broken Isles, which will earn Revered status with the Armies of Legionfall. With Mager Tower's buff: Reputable -Will increase Reputation gains with Armies of Legionfall. then you will get reputation rapidly upgrade. With the Darkmoon Faire and activity of World quests, it maybe more effective. 
If you have completed Broken Isles Pathfinder, Part One, it will be much easier for Unlocking Flying. If not, no worries, Part one and new achievements of part two can be carried out at the same time.
With the flight, a lot of things will become very simple, what do you think?