There's really no fast way of doing this.
Challenge has two parts. One is for landing 150 headshots with the bow and the other from doing 50 assassinations off a ledge.
I suggest picking one of the two tasks and then snapping the achievement app till you reach 50% and then start working on the other one.
One possible spot for doing Strike From Above is the stronghold in Durthang Outskirts. Alert the Uruks to your presence and they should call more. Run down the hallway where the Caragor cages are. Then climb up one of the two archer towers and then SFA to a enemy below (Hold
next to the edge and wait for the "
Stealth Kill" prompt.) and then climb up the other tower and repeat. This method isn't fast but it is consistent. Enemies should have yellow arrows above their head, so you may need to back away from the ledge if they stay red as you'll usually only be prompted to knock them down.
Next is the headshots. Hold the
to aim and then hold
to power up your shot, release the
once you have your reticle on their head.
I suggest equipping runes that boost your focus and headshot damage. Avoid runes that give just general damage because the game has no indicator that you killed the enemy with a headshot. So my shots are too weak to kill in one hit unless it's to the head.
In order to keep up your focus and Elf shot arrows I suggest riding around on Caragors and draining when you need to replenish.