To kill a Will-o-the-Wisp in The Elder Scrolls IV: Oblivion (including Oblivion Remastered), you must use specific types of damage because they are immune to normal attacks. Here are the effective methods and tips:

Effective Ways to Damage Will-o-the-Wisps

• Magic spells : Use elemental damage spells (fire, frost, shock, etc.) from a distance to avoid their health and magicka draining touch attack.
• Enchanted weapons : Weapons with magical enchantments that deal elemental or magic damage can harm them.
• Silver weapons : Silver swords or silver arrows are effective.
• Daedric weapons : These high-level weapons also work against them.
• Hand-to-hand combat : Only if your hand-to-hand skill is Journeyman level (50) or higher.

Combat Tips

• Will-o-the-Wisps drain your health and magicka when they get close, so keep your distance or retreat when you see the draining effect (a visible energy line).
• For melee fighters, learn their attack pattern: hit once, then back off before they drain your health.
• Using the Reflect Spell ability or potions can turn their drain attack back on them, healing you instead.
• Silver bows and enchanted arrows allow ranged players to safely attack from afar.
• If you lack the right weapons or spells, avoid engaging them as normal weapons do no damage.

Additional Info

• Upon killing a Will-o-the-Wisp, you can loot Glow Dust , a valuable alchemy ingredient.
• Will-o-the-Wisps are commonly found in caves, near Azura's Shrine, or during certain quests like the Fighters Guild questline.

In summary, to kill a Will-o-the-Wisp in Oblivion, use magic, enchanted, silver, or Daedric weapons, keep your distance to avoid their draining attacks, and strike strategically if melee fighting