Minecraft wiki entry for Defeating_a_woodland_mansion, associated text: Procedure: Climb to the top of the mansion where you’ll set up camp. From there, destroy the roof and whip out your ranged weapon of choice to shoot any illagers from above, room-by-room. After clearing a loot room containing illagers, use your water bucket to head downwards, extract the loot and head to the roof again for more shooting. This method is great in the sense that all secret rooms are guaranteed to be uncovered.