The fastest way to kill boss Leiden from Ab'Dendriel (Cults of Tibia Quest) is heling this boss by casting Mass Healing and using Ultimate Healing Rune.
Here you can see how it works:
It is one of the possibile ways to kill Leiden, but you can also kill it without healing the boss.
You can also use healing spells and runes while fighting with Lord Azaram from The Grave Danger Quest.
Lord Azaram changes his stage from time to time into Azaram's Soul assisted by few Tainted Soul Splinter . To kill the boss you need to kill Soul Splinter and lure Azaram's Soul into it's corpse (purple flame). To make the fight easier you can simply heal Azaram's Soul.