Easiest to get this achievement from one of the Forge Towers after you fast travel to it. The one I used was in Udun, in the bottom right of the map, here:
If there are any Uruks directly below, have at 'em. If they're not close enough, use the Distract ability (aim with your bow, and press 'G' by default) to move them closer to the tower and then rain death from above.