To get the Lifewarden's Breastplate, there is a series of quests you need to complete, ending in an elite who you will need a small group to complete.
The Archmage's Staff is a quest in Netherstorm which you receive from Ravandwyr in Area 52. You will have to complete this quest, the next step (Rebuilding the Staff) and then the third step (Curse of the Violet Tower) to gain access to the actual chain which ends in the breastplate.
This quest is called Malevolent Remnants, is from the npc Custodian Dieworth, and is six steps long. The last quest is called Destroy Naberius and, after you kill him, you are rewarded with your leather healing chestpiece. Congratulations!