Many mobs in Advent of Ascension used to have attributes that made them immune to certain types of damage. The attributes appear in the form of icons above the mob, togglable with the 'n' key by default. These were removed in 3.4.
Melee Immunity: Immunity to melee attacks.
Ranged Immunity: Immunity to all Bow, Archergun, and Throwing Weapon damage.
Blaster Immunity: Immunity to Blaster damage.
Gun Immunity: Immunity to all Gun, Cannon, Sniper, and Shotgun damage.
Magic Immunity: Immunity to all staves.
Fire Immunity: Immunity to all fire damage.
Explosion Immunity: Immunity to all explosion damage.
Status Immunity: Immunity to potion effects.
Special Combat Entity: Entities with unique resistances/immunities.
Hunter: This mob is a hunter mob and can only be damaged by the player when they reach certain Hunter levels. Hunter mobs that spawn naturally will not attack the player unless the player is the correct hunter level, but hunter mobs that spawn from spawners will attack players regardless.
Most Hunter mobs can be damaged through environmental damage regardless of the player's level, so fire, lava, and the like can be used to eliminate them. However, Hunter mobs will not drop rewards or Hunter experience unless killed by a player or their minion, both of which require that the player has a sufficient Hunter level.
Green sword pairs above their heads mean that the player looking at the mob has a high enough Hunter level to damage them. A red sword pair indicates that the player cannot damage the mob. In Creative Mode the player and the minions can always damage the mob regardless of the skill.