To kill a Will-o'-Wisp in Dungeons & Dragons effectively, the best approach involves overcoming its invisibility and high defenses, then dealing reliable damage:

• Negate Invisibility: Use spells like Glitterdust or See Invisibility to reveal the Will-o'-Wisp, as it frequently turns invisible to avoid attacks and escape

• Use Magic Missile or Force Damage: The Will-o'-Wisp has a very high Armor Class (AC 19) and resistance to most physical attacks, so spells like Magic Missile that automatically hit are highly effective

• Exploit Damage Types: It is vulnerable to force, psychic, and radiant damage, which bypass its resistances. Repeated hits with these damage types will wear it down

• Grappling Tactics: Although difficult due to its high Dexterity, grappling can prevent it from escaping. The Will-o'-Wisp lacks combat reflexes and fly-by attacks, so a coordinated party grapple can work

• Poison and Debuffs: Its low Strength and poor Fortitude saves make it susceptible to poison and conditions like Stinking Cloud that cause nausea, which can hinder it significantly

• Avoid Physical Melee Without Magic: Nonmagical weapons are largely ineffective due to its resistances and invisibility, so rely on magical weapons or spells

In summary, the strategy is to first make the Will-o'-Wisp visible using spells like Glitterdust , then attack it with reliable, magic-based damage such as Magic Missile or force/radiant damage spells, while possibly using crowd control or grappling to limit its movement

. This approach counters its high AC, invisibility, and resistances, leading to its defeat.