Dungeon Spirit

Nothing in the world produces one on its own, and no item summons it. It appears at the spot where something else in the Dungeon died, and only when a short checklist lines up: Plantera has to be down, the kill has to happen in front of natural dungeon brick wall rather than wall you placed, the victim needs more than a hundred maximum health, and it has to be an enemy that drops coins. That last rule quietly disqualifies statue spawns and anything the Old One's Army sends, which is why an otherwise perfect arena can produce nothing.

The health requirement is the part that shifts between modes. In Classic a few of the weedier Dungeon residents fall short and never release a spirit. Expert inflates everything down there enough that almost the whole population clears the bar, with Cursed Skulls the last holdout, and Master pushes even those over. So the same farm is meaningfully faster on a higher difficulty, before you count the extra drops.

• Pick a large room with several corridors feeding into it and hang a Water Candle there. You want a constant stream of qualifying kills in one place, not a tour of the whole Dungeon.
• Spirits fly through blocks and wander off fast, so a homing weapon like the Piranha Gun earns its slot. It latches on the moment one appears instead of leaving you to chase a ghost through the floor.
• Your luck feeds into the roll, so luck sources are not wasted here.
• Summoning King Slime inside the Dungeon on Expert is a known shortcut, since the slimes he splits into are themselves fat enough to qualify.
• The Ectoplasm goes into the Spectre set and its weapons, and into the Pumpkin Moon Medallion and Naughty Present, so work out how many events you plan to run before deciding you have enough.

If you carry a Wisp in a Bottle the two look nearly identical in a dark hall, and you will swing at your own light pet more than once. Dye the wisp and the problem disappears without giving up the light.