You'll encounter a Plyusch during the boss fight encounter within the Exhibition Complex. During mission "Tremors" while looking for Claire's left arm. Non-missable.Resistant's:
As Plyuschs are resistant to gunfire it's recommended to use a melee weapon during this boss fight. Ensure you upgrade one at the upgrade station on the stairs. Do your best to keep him at range so he can't melee/lunge at you. I found putting the incubation pod between me and him worked effectively. Allowing me to charge up a heavy attack before stepping round it to hit him.
At around 50% health he'll pounce on you triggering a quick time event with 4 different button presses. Continue to rinse and repeat the melee steps above. Once he's died the achievement should pop.