Merciless

Introduced in Generation VII, Merciless guarantees that every attack against a poisoned target lands as a critical hit. Whether the opponent is suffering from regular poison or badly poisoned (from Toxic), all damaging moves from the Merciless Pokemon will critically strike, regardless of the move's innate critical hit ratio.

This provides a consistent 1.5x damage multiplier against poisoned foes while also ignoring any Defense boosts the target may have accumulated, as well as any Attack drops applied to the Merciless user. The combination makes defensive setup against a Merciless attacker largely futile once poison is established.

Found on only 2 Pokemon -- Mareanie and Toxapex -- both Poison/Water-types with natural access to Toxic Spikes. This makes the poison-then-attack strategy entirely self-sufficient: set Toxic Spikes on entry, and every subsequent attack automatically critical hits any grounded Pokemon that switches in and gets poisoned.