After completing the mission entitled “
Unwarranted Aggression”, Talion will remain mounted upon a dominated Wretched Graug. At this time, begin traveling across Nurn with Wraith Vision activated until the red outline of a Uruk Captain becomes visible. As soon as you spot your target, tap the right thumbstick in to mark the Captain and start heading over to his location.
When you are near the target, hold the
to aim the Graug’s reticle and then hold the
to release a stream of Projectile Vomit. If you are able to hit the Captain with this vomit, then
Wretched Retch will unlock right away.
EDIT:[Posted by Pmalone6] It should be noted that this is received in the Lord of the Hunt DLC but otherwise great guide, thanks!