In order to achieve this, you will first need to purchase the "Strike From Above" ability from the upgrades menu. It is the third ability from the left in the top row.
Fast travel to the forge tower in the Barrows of Udun region. In the SOUTH WEST corner of the base of this tower an Uruk always seem to spawn close enough to the tower that you can use the ability on him. Approach the edge and look down at the Uruk.
Hold to target and then
press to strike.