Walk the Plank
Uncommon32.2%ObjectiveOptionalAnywhere
Triumph over a pirate invasion, a group of pillagers from the sea out for your booty... and your life!
Pirates invade randomly in hardmode, or on demand with a Pirate Map (dropped by anything in the Ocean biome during hardmode). They're a bruiser goblin army: fight at base with a choke point and something area-of-effect. The loot table is the draw - Coin Gun, Lucky Coin, Pirate Staff - though the achievement only wants the invasion beaten.