Exterminator Trophy

Vokh guards are first introduced in level 22; "Vokh Patrol". To kill a Vokh, you must shoot them from behind. They will charge at you the second that they see you, so you must teledash through them () to avoid their charge attack. Once you successfully dash through them, quickly shoot at them from behind to successfully kill them.

The best level to farm Vokh kills is in the 22nd level, "Vokh Patrol". This level has 7 Vokhs. You can choose to replay the level until the trophy unlocks. This is also a perfect opportunity to master the level for the "Perfect" medal. This trophy is cumulative. If you choose not to farm kills, then killing Vokh across all levels will count towards the trophy. The trophy will unlock when you kill the 50th Vokh.
 

(This is an image showing what a Vokh looks like.)