Mithril dragons have a 100% drop of dragon bones and three mithril bars, in addition to dropping very good High Level Alchemy items. While they are fairly difficult to kill due to their high damage output, using Soul Split in addition to good defensive armour can keep you at or near full health while killing them. Mithril dragons are weak to earth spells, although with a chaotic staff and a high Magic level, any type of spell will have 100% accuracy against them. Super antifires are a must -- otherwise using a anti-dragon shield or equivalent will lower your damage too much.