Summons atomos adds in several patterns. Move away
The orb he fires transfers to the matching colored Atomos, and then detonates. A blue orb must be run away from; a gold orb must be stood on top of.

Summons hand adds that tether to a target and need to die before they get to them
If whoever is tethered faces the hands, the hands will be petrified. Otherwise they chase you and eventually kill you.

There's an attack that turns the floors to shadows and puts a shadow debuff on everyone
Stand still when you hear about the floor being covered in shadows. If you do this, you won't be affected.

Boss 4
Watch out for a mechanic where you have to stack in to split damage, but ALSO need to face away from a player.

Also: When meteors are falling, the center meteor requires either someone heavily mitigated (like adlo/stoneskin here), or a tank. It does way more damage than others.