This is what Cray has to say. I'll just post his information on all four phases.
Has a permanent Backlash that reflects 15% of damage taken as Light damage and 15% of damage taken as Darkness damage.
Skill deals -25% damage, applies 1 turn of Light Imbue or Darkness Imbue to the player at random. If there are any negative statii on the mob, this attack deals a further -25% damage, cleanses all negative status effects on the mob, and then Dazes the mob for 3 turns, with a base stun rate of 10%, and a further 10% stun rate per negative status that was on the boss when the skill fired.
Has Regen worth 50% of a mob attack per turn. If it's ever brought below 2/3 or 1/3 of its max HP, any time it's above those values it'll deduct HP to be below that value, AKA it can't regen past each third of its HP once knocked below it.
When using its skill, it loses Freedom for the turn, and charges up for a turn. During this charge turn, if it gets hit 4+ times, or gets hit with a stun status (so Paralyse/Freeze/Daze, anything that CAN make the mob skip its turn), the charge gets disrupted. If successfully charged, the mob does a Void attack worth 300% normal mob damage. If the charge is disrupted, then the mob will skip its turn (basically it uses a fiddly stunbreak), gain +20% Heal resist (AKA Heal element is less effective), and it gains +20% EleVuln to all elements for 3 turns. After either of these takes place, the mob regains the Freedom status.
Loses Freedom and Boss Boost, instead has an Unstoppable effect where it burns some SP to break out of any stuns. Also has 50% stronger SP generation, so it can cast its skill more often and you basically have to spam stuns to truly drain its SP.
Skill deals normal damage, but this damage is split between three hits, the first doing HP damage, the second x1.5 MP damage, and the third x1.125 SP damage.
When attacked, adds the element of that attack to a list. Each time it gets hit with an attack that's already on the list, it adds 1 charge. The list resets when the mob fires its skill.
Skill seeks Light/Dark/Harm, and deals +10% damage per charge currently on the mob. Skill also applies a permanent Regen to the mob worth 5% of a mob's attack per charge. So if the mob has 3 charges, the skill does +30% damage and applies a perm Regen to the mob worth 15% mob damage per turn. Once the skill is done the charges are wiped.
Regardless of which phase you're fighting that has Regen, having ways to deal a lot of damage within the confines of soft damage caps is helpful. Remember that you can switch out equipment every phase.