Defender of the Broken Isles

According to mmo-champ:

Broken Isles Pathfinder, Part Two was added in Patch 7.2! According to the latest developer interview, it will probably be a few weeks before anyone has earned it. You won't be able to complete Defender of the Broken Isles until invasions activate a week after the patch releases.

I was here looking for this info myself. See ya in next week!

EDIT:
Legion Invasions
The Legion's presence continues to grow in Patch 7.2, with invasions occurring around the Broken Isles.

• The invasion begins with a zone being under attack. The skybox, world quests, and the NPCs present are changed in some areas, similar to the pre-expansion invasions.
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• The invasion quests are similar to the regular world quests, requiring you to kill NPCs and click objects.
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• After completing all six invasion world quests, you have completed the Assault on Zone quest, allowing you to move to the next stage.
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• Next you are sent to a specific point in the zone to defeat more invaders, with some similar followup quests.
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• After all of these quests are complete, there is a three player invasion scenario that sends you to the Legion ship in the sky.
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• On the ship you defeat various NPCs before defeating the leader and taking down the ship.

*Whole info from mmo-champion about this achievement.

I don't know about any of the other post but it says at the top of this page "Legion Assaults do not start until the week of April 4th." so there isn't anything you can do until then.

As of the latest hotfix, this achievement is no longer required for the broken isles pathfinder part two achievement.