From Diablo Wiki
Shielding is a Boss Modifier that may appear on Champions or Elites in Diablo III.
Shielding causes a shielding bubble to appear around the Champion, boss, or boss' minion for a few seconds, every few seconds. The shield grants immunity to all forms of damage and reduces or deflects many CC effects as well, for the duration of its presence. Monsters can not be hurt in any way while shielded, though they can still be slowed by some rooting effects. (Ones that have a general area of effect like Caltrops, but not specifically targeted ones like Entangling Shot.)
Shielding is identical in function to the (removed) Invulnerable Minions effect, except that one only appeared on boss minions, and was permanent. Shielding pops in and out on Bosses and Champions, turning on for a few seconds, then vanishing again without warning or any set on/off time.
- Monster Level Minimum: 31 (Nightmare and later)
- Available to: Champions and Rares and their minions.
- Damage Type: n/a
- Additional Resistances: n/a
- Can Not Spawn With: Avenger, Extra Health, Health Link, Horde, Illusionist, Invulnerable Minions, Missile Dampening, Vampiric.
Magda has a shielding-like property, and her shield provides her with limited invulnerability, but the on/off timer for the shield is based on events during her battle, rather than triggering on a semi-random timer.
Patch v1.0.4 in August 2012 nerfed the shielding property as follows:
Shielded monsters can not be damaged, so players should learn to recognize the effect and not waste time attacking a monster while it's invulnerable. As is often the case, this modifier is more dangerous when found on Champions, since all of them have it. Bosses and their minions get shielding, but it seems to be less frequent, especially on the minions.
Slowing effects that rely on hitting the enemy, such as Entangling Shot, or stunning attacks like Seismic Slam are generally ineffective against shielded enemies. Area effects like Caltrops may still work though, so adjust your tactics accordingly.
Mercifully, Shielding can not pair with Invulnerable Minions (since removed) on the same boss and his pack.
 Life Leech and Life on Hit
Shielding can be most dangerous to characters who survive by drawing constant health via Life Leech or Life on Hit, since the shielded monster takes no damage and thus yields no leech or LoH. Players thus need to be sure they're not targeting the shielded monster and gaining no health from their hits.
The Shielding bubble has a greenish-tint and is instantly recognizable when in effect on a monster. Boss minions will also gain a shield at times, and their tooltip shows "Shielding Minion" whether the shield is active or not.
A Barbarian struggles against a Shielded boss in v1.0.3,
|Monsters of Diablo III Beasts Beasts: Superunique Demons Demons: Superunique Humans Humans: Superunique Undead Undead: Superunique Objects Bosses Boss Mods Monster Info Fanmade|
|Primary Quest Uniques||
Act One Bosses:
Skeleton King -
The Warden -
Strong CC (Limit 1): Knockback -- Nightmarish -- Vortex -- Jailer
|Act One Uniques||Araneae - Web Spit||Leoric - Summon Skeleton, Teleport Strike, Triple Cleave||The Butcher - Smash, Grappling Hook, Spear, Frenzy Charge|
|Act Two Uniques||Maghda - Forcefield, Summoning Ritual, Punish, Moth Dust, Teleport.||Zoltan Kulle - Fiery Boulder, Time Bubble, Ceiling Collapse, Energy Twister,Vanish, Creepy Laugh||Belial - Phase One: Triple Fireball, Summon Vipers. Phase Two: Poison Meteors, Ground Pound, Lightning Strike, Lightning Breath.|
|Act Three Uniques||Ghom - Acid Slime, Sneeze, Gas Cloud, Chomp.||Siegebreaker - Triple Hit, Charge, Leg Stomp, Grab and Throw.||Cydaea - Pain Bolts, Web Trap, Spider Pits.||Azmodan - Bellyflop, Laser Attack, Falling Corpses, Demon Gate, Pool of Destruction, Globe of Annihilation.|
|Act Four Uniques||Rakanoth - Cleave, Teleport Strike, Blade Strike, Volley, Summon Minion.||Izual - Frost Nova, Frozen, Summon Minions.|| Diablo - Phase One: Claw Rip, Charge, Hell Spikes, Shadow Vanish Grab, Curse of Destruction, Ring of Fire, Ground Stomp, Curse of Anguish, Curse of Hatred. |
Phase Two: Shadow Clone. Phase Three: Overdrive, Lightning Breath.