AA - meteors (like in ICP vet hardmode, they do moderate damage and stun) and lighting atronach adds that enrage in execute phase like the first boss.
Hel Ra - extra gargoyle statues that start attacking after boss' Shehai Storm (or whatever that attack is called), unless you break them in statue form, by aiming boss' shield throw at them. Also its possible to just stack and aoe them down (can be a bit risky). Also boss can turn a random player into statue, and that player will get a synergy promt, but using that promt instakills everyone who stands near said player. Shield throw from the boss also breaks the statue and sets the player free, so it makes sense to stack at affected player to trigger shield throw, or, if its a tank, run to another side of the boss' arena and let them use synergy.
Sanctum - poisonous projectiles, most notably World Shaper (an instakilling aoe aiming at random player). So when the boss raises his hand and starts casting, everyone must be prepared to run/dodge that attack, it looks like a huge green bubble.
Maw of Lorkhaj - This hardmode is still not conquered.