At the start of the fight, Lorroakan will begin siphoning power from his four myrmidons, granting him immunity to Acid, Cold, Fire, Lightning. If he is attacked, he will use Elemental Retort on the person who attacked him, dealing significant damage from 4 different elemental types, one for every myrmidon he is siphoning from. Elemental Retort doesn't cost a Reaction, allowing him to use it over and over again each round.
On its first turn, the Earth Myrmidon will cast Stoneskin on Lorroakan, giving him extra resistances to non-magical damage.
If the party saved Rolan from his expedition into the Shadow-Cursed Lands in Act Two, but did not save Cal and Lia during Rescue the Tieflings, then Rolan will choose to fight alongside Lorroakan instead of abandoning him to help the party.
Tactics
Ignoring Lorroakan and killing his Myrmidons first is a possible strategy. Each killed Myrmidon removes the associated elemental immunity and damage from Elemental Retort, making it less punishing to actually attack him.
Alternatively, it is also possible to simply tank the damage with your strongest martial characters and aim to kill Lorroakan outright as he only has 98 HP on Balanced difficulty and is the most dangerous enemy with his array of spells. Given his 4 different damage immunities it may be easiest to deal this damage through weapon attacks, especially if the character in question has equipment to enable critical hits.
Tactician difficulty
On Tactician, he gains Elemental Retort for acid, fire, lightning and cold elements, which ignores resistance and slightly increases damage. This can make his Elemental Retort more powerful.
Notes
Lorroakan's myrmidons have weaker weapons compared to other myrmidons encountered throughout the game. They don't have innate elemental damage components and are not enchanced. The unique weapon abilities aren't changed.