Buffy picks up their glow from hundreds of feet away - long before they can see her. Then she hits them with a combo spell that both immobilizes them and busts their poison immunity. She then immediately chases the spell with a poisoned arrow. Between the spell and range, the wisp has no idea where to seek the source of their pending death. When the glow extinguishes, Buffy moves on; one cannot recover arrows from wisps, and their glowing dust is hardly worth the long hike to track down their slippery green goo.
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