I Was Framed!

At Darktide Roost pull a hostile [npc=78328] mob next to a [npc=84927] - a full healthed whelp, not the injured ones - where there is a [npc=84002] nearby and then let the flamethrower damage the whelpling until it call for its mother - you shouldn't touch either the whelpling or the robot in any way! - The nearby [npc=84002] will then descend and start attacking the robot, granting you this achievement immediately.

Multiple reports indicated that any kind of offensive companions / passive skills (warlock pets, bodyguard, lightning shields, maybe hunter pets too?) can mess up the achievement from triggering when they automatically damage the mobs unnoticed.

Also, as Scyllaxs noticed below, you may also have problems to complete this acm while mounted.

I was going crazy here! I though you had to be DRIVING the Darktide Rylakinator-3000 like for the You Have Been Rylakinated!. And the flame thrower kept killing the stupid [npc=84927], so I was loosing my mind. THEN I figured out (and it took me far too long) that I should drag one of the hostile shredders [npc=78328] TO the whelping and place it so that ITS Flame Thrower hits the whelp. Oh my god. So easy.

58.8. 89.4 is an ideal spot so long as no one else is around. Its just in front of the cave where Nagidna (rare) spawns.

The spot is just next to a Darktide whelping, and in between a Darktide Rylakinator-3000 and a Darktide Matron.

Facepull the Rylakinator and kite him only about 10 meters to the whelpling. His flamethrower will hit the whelping and the matron will come and attack him and trigger the achievement.