Although Blizzard just pushed a hotfix to cycle Legion Assaults significantly faster, completing 10 events would still normally take three to four days. However, the achievement increments each time the scenario's objectives are completed, rather than when the quest is turned in. This means that players can speed up the process by repeatedly abandoning the Assault quest and then re-accepting it to queue for another scenario.
Find an active assault.
Complete the four world quests and initial chain.
Start the final assault scenario: Battle for Azsuna, Battle for Val'sharah, Battle for Highmountain, or Battle for Stormheim.
Complete all steps up to the final objective (Get Out of There, Time to go, or Escape!), in which the quest is marked completed.
Instead of taking the mount out, simply leave the group and abandon the quest without turning it in.
Re-accept the quest, and queue again.
A bit tedious perhaps, but sped up even more by the fact that Legion Assaults no longer require a group, meaning the achievement can be quite comfortably finished in no time even while completely solo!
Be sure to check the achievement tracker for credit before leaving the scenario!
Finishing all 10 assaults awards a Vertical Felsteel Chain and unlocks the Hanging Felsteel Chain for purchase from the Legion Remix housing vendor. Along the way, you can also complete the Defending Broken Isles portion of the Legionfall campaign, as well as Defending the Broken Isles I and II for a little bit of knowledge.
