At the start of the fight, Lorroakan will be siphoning 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 doesnt cost a Reaction, enabling him to use it over and over again each round.
This makes ignoring Lorroakan and killing his Myrmidons first a possible strategy. Each killed Myrmidon removes the associated elemental immunity and damage from Elemental Retort.
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 with his array of spells.
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.
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.