Mummies are the bigger common enemies that replace the the Heavy enemies in the main game world.
They're only found in afterlife areas, the final blow must come from fire damage, either by using a Ranged or Melee weapon or by using your torch when he is really low on health.
best method is a bow with a fire mod, avoid headshots, let the burning effect wear off before hitting them again. It is also wise to pick off the smaller enemies around them if you accidentally get too close and get them all after you.