Death Knight Cavalier Bugged.

NPC Death Knight Cavalier in Naxxramas (http://www.wowhead.com/npc=16163) seems bugged. His shield Bone Armor doesn't drop (http://www.wowhead.com/spell=55315) We did more then 50k damage for sure Donor rogue and donor DK. But the shield gets to 30 seconds and ressets. You can use the Killing Spree in my action bar for timer to see when what happened.

 

I've come across quite a few of those bosses/mini bosses/quest npcs that just seem to reset back to full life/mana as soon as they dip below 20-25% hp.

However, I found that by sticking to the fight and whittling them down over and over again, they eventually stop spontaneously regenerating.

The number of times you have to start over again seems random as I've killed many of them repeatedly (Bronjahm - http://www.wowhead.com/npc=36497 - in Forge of Souls immediately comes to mind).

Hunker down and stick to it and they'll eventually stop regenerating.