Astral Attire

Right now in season 3 you can earn this achievement by purchasing a full set of honor armor from Velerd at 55, 76 in Dornogal for 6475 honor. You can see the armor set under Dunewalker <Armor Type> -> Aspirant. If you buy a piece of honor armor you can click on the message in your chat to see the set.

Gotchas:

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• To make the achievement pop, you must equip each piece, such that they are no longer refundable.
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Tips:

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• If the pattern from previous expansions holds, once season 3 ends, this achievement will become unattainable until the next expansion, when this gear will be purchasable for Marks of Honor. (Since there is no season 4 this expac, it's possible there will be no gap during which this is unobtainable, not sure.)
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• If you are looking to get easy honor, the Comp Stomp Brawl is a good choice, when it's active.
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• Only one of the back piece completes your set- if you want to spend the minimal amount of honor, check which it is before you buy a back piece, by buying everything else first!
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Make sure that you bought proper cloak. There are few of them and they are from different sets. And ensure that you equip item, so refund option goes off.

I got this achievement for my Druid purchasing the following items from Velerd in Dornogal PVP area 55.0,76.5 for a total of 6475 honor :

Astral Aspirant's set :
Leather Helm
Leather Spaulders
Drape
Leather Vest
Leather Wristwraps
Leather Gloves
Leather Belt
Leather Breeches
Leather Boots

Once all equipped, tada, achievement. Though a friend had to log off and on again to get their's.