Magma Mephits are near the Adamantine Forge in Grymforge at X: -556 Y: 277. Some of them ambush the party near a Mithral Vein, which contains one piece of Mithral Ore.
If the creature is only wearing metal armour, it always receives Disadvantage.
If the creature is still touching the metal, you can use a bonus action on subsequent turns to deal another 2d8Fire and force the creature to let go or receive Disadvantage.
Magma Mephits are quite weak individually, so they make up for it with their mobility using flight and by attacking in large numbers. Considering their immunity to Fire and ability to fly, they can use the environment to their advantage. Their Fire Breath attack can also create more hazardous terrain with Fire surfaces, likely setting anyone struck aflame as well. Even killing a Fire Mephit can cause harm, as their Death Burst can quickly stack up damage.
An effective counter is to discover and lure them out, then defeat them at range (e.g., an Arcane Trickster who is Hastened and has the Alert feat, using Minor Illusion in conjunction with Enhance Leap and possibly Feather Fall). Preparing for Fire damage is also key. Using an Elixir of Fire Resistance halves any Fire damage and grants immunity to Burning until the next long rest. Also, simply using a bottle of Water to gain the Wet condition is a simple and cheap method for protection, though its short duration limits its usefulness. It can at least easily put out a party member who is Burning.
As for combat, spells and attacks that deal Cold damage are potent against Fire Mephits, with the spells Ray of Frost and Ice Knife being cheap yet effective means to kill them. Arrows of Ice are a more physical method of dealing Cold damage. Relying on ranged attacks and spells is the best way to avoid damage from their short-ranged Death Burst as well. Overall, Mephits aren't too hard to hit and kill thanks to their low AC and HP.
Another way to avoid their Death Bursts, especially with melee attacks, is to either shove them away before dealing the killing blow or to Toggle Non-Lethal Attacks to knock them out, since knockouts do not activate their Death Burst. However, ranged attacks, spells, and critical hits bypass non-lethal attacks.
