Baleful Harvest

The Pumpkin Moon summons with a Pumpkin Moon Medallion (30 pumpkins + 5 ectoplasm + 10 hallowed bars at a mythril anvil) at night, and runs waves until dawn - wave 15 is the target. It's a DPS check: post-Golem gear, a proper arena (lava trenches, heart lanterns, honey pools), and summon the moment night falls to leave time. Mourning Wood and Pumpking drop the event's excellent loot; wave 15 pops the achievement even if dawn cuts short your victory lap. Its winter sibling is Ice Scream.