Letter of Marque

In a map with minor factions enabled, you'll see pirate ships in the gravity wells around the pirate homeworld and you should keep killing them until you reach 100 killed. Pirates don't venture further than this, so if you haven't seen any yet then keep exploring/expanding first.

Killing them doesn't affect your ability to reach max reputation for Peacemaker.

This is soft-missable because if you or another faction destroy the pirates first then there won't be enough ships left over in the match to get this to 100.