Despite their combat level, Black demons have fairly low defence. However, they are usually one of the most dangerous monsters in the dungeons they inhabit because of their accuracy and high max hit. Note: that when planning to train prayer with the ashes that are dropped, it is advised to bring infernal urns as this will grant more experience than just spreading the ashes.
While using melee used to be a cost-effective method of killing them, due to their new magic attack and its accuracy, many players planning to melee will find they are getting damaged often. It is recommended to use protection prayers to reduce some of the damage. You should also have good armour, food, and have a defence level of at least 80. A healing familiar and ring of life are optional although highly recommended. Prayer flashing can also be used to reduce consumption of prayer points, but requires fast reflexes and good timing. Finally, those with level 92 Prayer can utilise the Soul Split curse to heal slightly with every damaging hit.
Guthan's set-effect is useful when training on Black Demons. Please note that the demons can damage you very fast and you may not be able to heal fast enough unless you have 80+ Defence. In case the effect of the armour set can't heal enough on the black demons, there are usually weaker monsters nearby that the player can heal on, such as wild dogs or earth warriors.
The Darklight is a very cheap and effective weapon against Black demons if you have fairly high stats. It is especially helpful if you are on a task and use your Slayer helm.
Items: 4-6 Prayer pots, Super or Extreme sets, 2-3 Prayer renewals, 10-15 pieces of food, a summoning potion or super restore(if you have familiar).
Ranging black demons is the most effective way to kill black demons. Post-EoC, they have gained a magic attack ability, which makes safe-spotting tactics obsolete. As black demons are weak against bolts, one effective way of killing them is by using the Black salamander or higher grade range weapons.
Magic is not recommended due to black demons having high defence against magic, leading to the low speed and max hits of magical attacks slowing down kills in comparison to using range unless one's ranging level is extremely low. In addition, the cost for runes can add up to losses over time, even when collecting the dropped infernal ashes. The lone exception to this is the Polypore staff, especially at higher magic levels, because of its cheap usage cost, high maximum damage, and the profitability of infernal ashes. When combined with a Extreme magic/Overload potion, an arcane stream necklace and a Hexcrest/Full slayer helmet (on slayer tasks), the Polypore staff can yield fairly fast kills.
Use of the Dwarf Multicannon is not recommended. Since the EoC, the cannon is no longer effective against Black Demons. Using the cannon would also cause many demons to attack you.