You can only start getting this during barry's campaign episode 2 here's a place to start farming your 20 kills unless your stubborn (@The Dauntless) then enjoy getting them through out your play through
After the cut scene where Barry says my vision is getting bleary and Natalia says there's a monster ahead don't you see it. Aim forward and start shooting with what ever gun you like, once you kill it press
and then retry, you will load right before you walk into the cut-scene, just skip the cut-scene rise and repeat and enjoy farming your 20 kills while you listen to music or podcast (normal what i start doing during farming situations)
Hope this helps and if you have any other questions leave a comment or private message me, thanks