To kill a Will-o-the-Wisp in The Elder Scrolls IV: Oblivion Remastered, you need to use damage types other than normal physical attacks, as they are immune to standard weapons and poison. Effective methods include:

• Using magic spells (elemental damage such as fire spells work well).
• Using enchanted weapons that deal magical or elemental damage.
• Using silver or Daedric weapons.
• Hand-to-hand combat if your skill is at least Journeyman level (50 or higher).

Melee attacks with non-magical weapons like iron or steel will not harm them. It's advisable to attack from a distance with spells or enchanted bows to avoid their health and magicka draining attack when they get close. Reflect Spell effects or potions can also reverse their drain effect back onto them, making the fight easier. Once defeated, Will-o-the-Wisps drop glow dust, a valuable alchemy ingredient. In summary, the most reliable way to kill a Will- o-the-Wisp is to use magic or enchanted weapons with elemental damage, or silver/Daedric weapons if you prefer melee, while maintaining distance to avoid their draining attack