Bug auto-heal ?

[GarlicBuutter]

Hello everyone, i played a few hours on the DK and when i fought with Onixia for the second time she began to heal herself really fast, (half hp in 2 seconds) and when i went in the tier 3 instance for weapon, the wolf just didn't die, i fight around 25 minutes with him and every time he healed himself so i didn't killed him, but the other mob died in about 2 minutes, bug or what ?

Thank you

(sorry for the bad english it's not my most familiar language)