Level 3 explosive ability is required to unload the full 16 rocket clip at one go
Since the Decimator tries to lock on to everything in front of you. the easiest way I have found to do this is to engage a group of enemies on a skyscraper (some buildings like the science center will have enemies spawn above) kill all but one enemy with other means. Then whip out that Decimator and lock the full clip on and fire! (Listen for sounds indicating a full lock on
An alternate method is to go right up into the face of a mech and it should only lock on to it when you ADS.
There is a Decimator available in Picker's Pen up north right next to the agent DNA collectable.
Update: You require explosive level 3 in order to fire the full clip of 16 rockets at one go. Thanks LAZY SHIKAMARUX
Update 2: Thanks Lyco499 for the exact location of a Decimator as well as a second method to easily accomplish this and clarifying the weapon lock-on confusion some may have.