Protip: "Serial Killer" achievement/penance for killing 20 consecutive enemies with headshots can be done with brainpop

[Arcuārius]

Just do 20 psyker brainpop kills in a row and you get it.

From the moment I understood the weakness of my flesh, it disgusted me. I craved the strength and certainty of steel. I aspired to the purity of the Blessed Machine.
Your kind cling to your flesh, as though it will not decay and fail you. One day the crude biomass you call the temple will wither, and you will beg my kind to save you.
But I am already saved, for the Machine is immortal...
...even in death I serve the Omnissiah.

[Gym_Leader_Myth]

Oh, thanks, does it matter if you have to use your melee at any point in those 20 brainpops?