Level 30 Slayer is required to kill Pyrefiends. Their weaknesses are Pearl bolts (e) with any crossbow (preferably Rune) and are weak to water spells. When killed as a slayer assignment, you'll receive 45 slayer XP.
Charm Drop Rates
The melee attack of the Pyrefiend has a Magic base to it, so wear armour with good Magic Defence (Dragonhide, etc.). Though, for some people (depending on your defence level), it may be better to wear metal armour (Rune, etc.). It's best to alternate between the two, and see which style suits you best.
A small fire demon.
Clue scroll (level-2), Dragon med helm, Starved ancient effigy
Impious ashes (100%), Coins (10-500), Jug of wine, Gold ore, Chaos rune (12), Death rune (3-5), Fire rune (30-100), Nature talisman, Chaos talisman, Staff of fire, Steel boots, Steel full helm, Steel hatchet, Mithril chainbody, Adamant med helm, Rune javelin, Gold charm, Crimson charm, Green charm, Blue charm