High chance by the end of the game you should unlock this, by chance if you don't any level has opportunities to boost this.
(Fast way to farm is to go to the third to last level,
when the level starts pause - un equip all your mods - un pause - then wait from your special gauge to fill - replace mods that boost the tempest skill and any other mods.)
Continue through the beginning till you encounter the crawling enemies, you will get a check point right before as you land so if you fail or die you will still have your special.
(Wait a sec till the enemies are off the wall - then gap jammer over them to the wall - when you land on the wall jump off right after - turn look down to them and blast them with tempest. Die and repeat,)
Atleast 12 killa per try. default looking sensitivity or higher needed for speed.