"Kill 5 Threshers" is an objective in the sidequest Perfectly Peaceful in Borderlands 2
Walkthrough[edit]
While this mission properly takes place in the Caustic Caverns, these optional objectives can be completed anywhere. Head over to the The Highlands and go to either the Blake Bridge and drop down to the dry riverbed below or to Lake Shining Horizons to find the Threshers you are looking for.
