Kill the additional creature, triggering Bastion of Remembrance and Pitiless Plunderer.
Resolve the Bastion of Remembrance trigger, causing each opponent to lose 1 life and you to gain 1 life.
Resolve the Pitiless Plunderer trigger, creating a Treasure token.
The Treasure token enters the battlefield as a 0/0 creature and dies due to state-based actions, triggering Bastion of Remembrance and Pitiless Plunderer.