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Every 50th kill of this enemy drops Elf Archer Banner.when she turns up
She has no natural spawn anywhere in the world. The only place to meet her is inside a Frost Moon, which you start yourself with a Naughty Present after dark once Plantera is dead. She begins arriving in the second wave and keeps coming for the rest of the night, and every one you kill pushes the wave counter along, so clearing them is never time taken away from the run.
the damage you do not notice
One archer is harmless. A dozen of them, while something large is already chasing you, is how a Frost Moon quietly falls apart. She fires from the far end of the arena and her arrows leave you burning, so your regeneration spends the whole night undoing the fire instead of refilling what you actually lost. Kill her on sight rather than saving her for the mop up, and do not stand still trading with the ground wave while she lines you up from behind it. If you have fought the Skeleton Archer or the Pirate Crossbower, this is the same pattern with a worse debuff attached.