Sandstorms in The Waking Shores

Open your map while in The Waking Shores and look for the active earth storm icon. You need to kill enemies that have the Elementally Imbued debuff. Storms are only active for a few hours, and multiple storms can be active across the continent simultaneously.

The fastest farming location is the underground complex at 66.7, 25.8, entered via the Qalashi Drakeflayer area. Note that half of the mobs above ground are not Elementally Imbued, but the Qalashi Drakeflayer below are all marked and spawn in groups of two to three at a consistent rate, allowing you to circle the complex continuously without waiting for respawns.

Alternatively, head to the large basement room at 67.19, 25, which has approximately 10 non-elite mobs and 1 elite that chain pull and hyperspawn. Clearing this area repeatedly can complete the achievement in around 10 minutes at high gear levels.

Lava Phoenixes and Lava Hatchlings are excellent targets. A large pack of phoenixes sits on an elevated rock at 65, 28.8. When a bird drops a feather on death and then rises again to be killed a second time, both kills count toward the achievement total — so do not kill the dropped feather, only the bird.

While waiting for phoenix respawns, run laps through Scalecracker Keep at 66.6, 26, where mobs respawn very quickly when fully cleared. There is also an additional pack outside the keep at 67.5, 27.2 that respawns more slowly, useful as a rotation stop between the keep and the phoenix rock.