Location:
Skull Cavern, all floors.
Strategy:
Mummies are slow but have high health and can revive after being defeated. To permanently defeat a Mummy, you must hit it with a bomb after it falls. Use your strongest weapon to knock them down quickly, then immediately place a bomb next to their fallen body. Explosive Ammo for the Slingshot also works. Be careful not to get surrounded. They are not weak to any specific element.
Rewards:
Cloth, Solar Essence, Void Essence.