Waterfiends are fast, easy, and campable though FireZ.
Good weapon for Waterfiends is Chaotic Maul. Then buy a Dwarven Helmet (highest crush bonus in game for helmet slot) and wear Karils/Armadyl armour for Magic and Range Defence and you are good to go.
Killing them for Charms I would use Uni to camp them pretty much forever, but if you are doing it just for a slayer task you can use Uni+Turmoil+Extremes and they are done in no time.