Some Elite/Champion mobs have a "Shielding" modifier where they bubble themselves for a while. How does the shield work; does it absorb a fixed amount of HP? Are they totally immune (from damage and effects) while encased?
"Monsters imbued with the Shielding trait have the ability to temporarily make themselves immune to all forms of damage". To quote diablo wiki.
It also just lasts for a fixed period of time, depending on how many people are in your party it can go from 3-5 seconds of invulnerability from all damage types.