Very easy and you'll likely unlock this through natural gameplay. To make uruks flee from Morgai Flies you simply have to shoot the hanging nests (they look like large bee nests) found around the map when uruks are near them.
Quick tips:
If you're having trouble finding the nests to shoot you can use your "wraith vision" (default is
) and it will highlight the Morgai Fly nests along with other things like uruks and caragor bait.
You won't need to shoot twenty separate nests, but rather cause twenty total uruks to flee from the flies. You can always gather a group of enemies together that are trying to attack you and lead them to a nearby nest to then drop on them if you're aiming to get this done quickly.