What the achievement asks
Obtain 5,000 Elemental Overflow. Account-wide.
It is a total, not a balance
You do not need 5,000 at once. The count is everything you have ever looted, so spending it as you go costs you nothing.
That matters, because Storming the Runway wants you to spend 3,150 of it on armour, and people hoard for weeks thinking the two achievements are in conflict. They are not.
Where it comes from fastest
Primal Storms pay it, but the concentrated source is the Forbidden Reach: killing rares and completing the zone's objectives. A couple of hours there covers the whole 5,000, which is far quicker than waiting on the storm rotation.
Where it fits
One of three under Into the Storm, a piece of A World Awoken. Since the Forbidden Reach is also a whole branch of its own in You Know How to Reach Me, the two farm each other.