Please note that the achievement tracker will not show any progress as you equip set pieces.
Legendary sets consist of six armor pieces: helmet, chest piece, legs, shoulders, gloves, and boots. All pieces of a set must be equipped at the same time for achievement progress to be made. Progress is account wide and not tied to a single character.
There are seven core sets that were available at launch, each with their respective achievements, that count towards this achievement. Any armor sets not specifically listed here were added to the game post-launch and will not count towards this achievement.
Four sets are class specific.
Musketeer Garb - Sharpshooter
Cosmic Garb - Railmaster
Celestial Garb - Dusk-mage
Mountain King's Garb - Forged
Three sets are region specific.
Woodsbeast Garb - Goblin (fire)
Skittering Garb - Hyvid (poison)
Arc-Powered - Voltura (electric)
All other sets not specifically listed drop in Fazeer's Dun'djinn or from the gambler.
It is unlikely you will acquire a full class set prior to reaching level sixty with a character. Grinding levels in Fazeer's Dun'djinn (the end game) that give the +50% gear luck modifier will be your best bet in polishing off any missing items. Region sets will drop in the respective levels in Fazeer's Dun'djinn. The chest pieces for each region set give a +20% gear bonus for a brief time after killing the monster associated with the region which is a boon when finishing off the region, or class, sets. Finally, all of the sets can be acquired through the gambler and gear bundles given through contract completion.
Due to recent updates, Fazeer's Dun'djinn is no longer the best way to gather legendary sets. All set pieces are now available in end-game dungeons making gathering specific region sets more difficult. Bosses now have an increased chance of dropping legendary items and the bonus gear luck modifier from playing on ridiculous difficulty has increased. To clean up region sets return to the areas available in the campaign and target bosses within the region that drops the set you're looking for. Most overworld levels have multiple bosses to kill. Ignore mobs and take advantage of phase beasts. Bosses will respawn after traveling to two other areas. The layout of areas will remain the same unless you exit the game. The Hyvid region has the most easily accessible bosses so clean up class sets there. Remember to take advantage of gear bonuses offered by the legendary chest pieces of region sets. There has been some reports that returning to campaign areas with a level 60 character decreases the chance of legendary items dropping. I cannot confirm this to be true, as I received comparable drops with both mid and max leveled characters.
All set pieces can be acquired through the gambler and gear bundles given through contract completion. With recent updates the chance of legendary items dropping from rare gear bundles has increased.
If you have any additional tips or corrections send me a message.